Why does your tooth give a sharp zap when you bite aluminum foil?
― A tiny battery forms in your mouth, just for an instant
The moment you bite down on a scrap of foil, a jolt like electricity shoots deep into your tooth. That's not your imagination. When aluminum, a tooth filling, and saliva all come together, a real battery forms inside your mouth. The current that flows is carried to your brain by the tooth's nerve, which reports it simply as "pain."
At lunchtime, you take a bite of a rice ball wrapped in aluminum foil. Without realizing it, you chew down on a small scrap of foil you hadn't peeled away.
At that instant, a sharp jolt shoots through the core of your tooth. It's not like the sting of cold water, and it's not like a cavity ache either — it feels electric.
Yet some people bite the same foil and feel nothing at all. What makes the difference? It comes down to whether that person has a metal filling in their mouth.
There are only two reasons
When metals with different properties — like aluminum and a silver filling — touch in the presence of moisture, electricity flows. It's exactly the same mechanism as the battery you build in a science class experiment.
The nerve at the core of a tooth reports every signal that reaches it — heat, cold, or electricity — as "pain." That's why even a tiny current turns into a sharp jolt.
In other words, the cause of the pain isn't the hardness of the foil or a sharp edge. It's a current that springs up for an instant inside your mouth. Let's go through it step by step.
Metals differ in how easily they give up electrons
Every metal has its own tendency to give up electrons. Aluminum is near the top of the "gives up easily" end, while gold and silver sit at the "holds on tightly" end. The bigger the gap between two connected metals on this scale, the greater the force pushing electrons along — in other words, the bigger the voltage.
Becoming a battery takes one more ingredient: moisture that can carry electricity between the two metals. Saliva, with its dissolved salts, fills that role nicely.
Look at Figure 1. The vertical scale shows how tightly each metal holds on to its electrons. The height difference between aluminum at the bottom and whichever metal you're comparing it to is, in theory, the voltage. Turning the dial to a different partner metal shows that the gap grows largest with silver or gold.
Fillings known as "silver teeth" and gold crowns contain metals such as silver, gold, and tin — all of which sit higher on the scale than aluminum. In theory, some of these combinations can produce a voltage greater than a single AA battery.
In an actual mouth, the thin oxide film on the aluminum's surface and other factors are thought to bring the voltage down far below this. Even so, it's still more than enough to stimulate a tooth's nerve.
The current reaches the nerve through the filling
The instant the foil touches the filling, the battery's circuit closes. At the point where the two metals touch, electrons move from the aluminum to the filling. On the body's side, charged particles dissolved in moisture (ions) carry the current from saliva and gums, through the tooth, to the filling. The arrows in Figure 2 trace that full loop.
In a normal tooth, the outermost layer — the enamel — acts as a wall that resists electricity, and enamel has no nerve in it. But in a tooth with a filling, the metal cuts through that wall and reaches close to the nerve. Because metal conducts electricity so well, the current is carried right up next to the nerve in an instant.
Inside the enamel, the dentin is packed with tiny channels — far thinner than a human hair — running toward the nerve. These channels are thought to be about one-thousandth of a millimeter wide, and filled with fluid. The sting of cold water is also believed to reach the nerve by traveling through these same channels.
And here's the curious part about how we sense things. The nerve at the core of a tooth can barely distinguish between heat, cold, pressure, and electricity. Whatever the stimulus, it all becomes a single kind of signal — "pain" — whose strength simply scales with the stimulus. Your tongue can tell the same current apart as "sour" or "tingly," but a tooth can only say "zap."
In dentistry, a very weak current is sometimes passed through a tooth to test whether its nerve is still alive. A living nerve responds to electricity too, with a "painful, tingling" sensation. The pain from foil is like unexpectedly undergoing this very test on your own, in your own mouth.
If your mouth has two fillings made of different metals, they can form a small battery between themselves in saliva. The same mechanism explains the tingle when a metal fork touches a silver filling. If pain continues to bother you, it's worth asking a dentist.
Summary
The pain from biting aluminum foil comes from a battery forming inside your mouth, made of three things: aluminum, a metal filling, and saliva. The filling carries the current close to the nerve, and the tooth's nerve — which can't tell stimuli apart — reports it as a sharp pain. People without fillings feel nothing because they have no partner metal to form a battery with.
That "zap" from the foil is the sound of a battery in your mouth.
To a tooth's nerve, even electricity sounds like nothing but pain.
You can read about the people who first confirmed that electricity could move a body in "Why did a frog's leg twitch the instant it touched metal?", about the different speeds of nerves that carry pain in "Why does the pain from stubbing your little toe arrive a little late?", and about how skin sensors can be fooled by molecules in "Why does mint feel cool when it isn't actually cold?".
- Wash a 1-yen coin (aluminum) and a 10-yen coin (mainly copper) thoroughly with soap and rinse with water.
- Place the 1-yen coin on top of your tongue and the 10-yen coin underneath, and confirm that neither alone produces any sensation.
- Gently touch the edges of the two coins together at the front of your tongue. If, the instant they touch, you feel a sour or tingly taste, that's the battery's current.
Be careful not to swallow the coins, and have young children do this with an adult present. If you have a metal filling, don't deliberately bite foil to test this. The difference between the "taste" your tongue senses and the "pain" your tooth senses is really the whole point of this article.
Want to know more? ― Terms, formulas, and links to the textbooksWe've marked which level each part belongs to, from middle-school science to university-level subjects
- Middle schoolCovered in middle school science
- High schoolCovered in high school "Chemistry" or "Biology"
- High school+Advanced high-school content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialist subjects (electrochemistry, dentistry, neurophysiology)
- ResearchNot yet settled even at university — something researchers are actively investigating
Middle schoolTerms: this phenomenon has names
- Chemical cell (battery): A battery made from two kinds of metal and an electricity-conducting solution. In third-year middle-school science, students build one with zinc and copper.
- Reactivity series (ionization tendency): The order in which metals tend to give up electrons and become positive ions. The bigger the gap between two metals in this order, the bigger the voltage.
- Dental pulp: The cluster of nerves and blood vessels at the center of a tooth — what's commonly called the tooth's "nerve."
- Galvanic current: The dental term for the current that forms between different metals inside the mouth.
Middle schoolHigh schoolCheck with a formula: how many volts does aluminum plus a silver filling make?
A battery's voltage can be estimated from the difference between two metals' "standard electrode potential" — a number that captures how tightly each one holds on to its electrons. Here's what each symbol means. All units are volts.
| Symbol | Meaning and unit |
| E | The battery's voltage (electromotive force). Unit: volts |
| E₊ | The value of the metal that receives electrons (the positive electrode). Here, silver. Unit: volts |
| E₋ | The value of the metal that gives up electrons (the negative electrode). Here, aluminum. Unit: volts |
| In symbols | E = E₊ − E₋ |
| In words | Battery voltage = value of the electron-receiving metal − value of the electron-giving metal |
| Where it comes from | Each metal has a fixed "how easily it gives up electrons" value (standard electrode potential); the difference between two metals is the force pushing electrons along. Electrons flow from the metal that gives them up easily (aluminum) to the one that holds on tighter (silver) |
| Aluminum's standard electrode potential | −1.66 V |
| Silver's standard electrode potential | +0.80 V |
| Tin's standard electrode potential | −0.14 V |
| Voltage of one AA battery | 1.5 V |
| Silver-aluminum battery (subtracting a negative becomes addition) | 0.80 + 1.66 = 2.46 V |
| How many AA batteries is that? | 2.46 ÷ 1.5 ≒ 1.6 |
| Tin-aluminum battery | 1.66 − 0.14 = 1.52 V |
| Silver-tin gap (two metals within a filling) | 0.80 + 0.14 = 0.94 V |
In theory, an aluminum-and-silver-filling combination makes a battery of about one and a half AA batteries. In an actual mouth, the oxide film on the aluminum's surface and components in saliva are thought to bring the voltage down well below this. Even so, because the metal reaches so close to the nerve, it's still enough to be felt as pain. The size of the current that flows is determined by the voltage divided by the electrical resistance of the path (Ohm's law).
High schoolHigh school+How a battery works, and why a tooth stings
High schoolchemistry teaches that at the negative electrode a metal is oxidized (aluminum gives up electrons and becomes a positive ion), while at the positive electrode something is reduced. On the positive-electrode side in the mouth, it's thought that the silver itself doesn't dissolve — instead, oxygen and other substances dissolved in saliva accept the electrons. Biology teaches that when a nerve receives a stimulus, it fires an electrical signal (an action potential) that reaches the brain and becomes a sensation. A current flowing in from outside can trigger this signal directly.
High school+The leading explanation for why teeth sting is the "hydrodynamic theory." It holds that fluid inside the dentin's thin channels (dentinal tubules) moves in response to temperature or drying, and the nerve deep in the channel senses that movement. An electrical stimulus, by contrast, is thought to shake the nerve directly, without going through this fluid movement.
UniversityThe "mouth battery" through the lens of electrochemistry
The actual electromotive force isn't simply the difference in standard electrode potentials. Shifts caused by ion concentration and temperature are described by the Nernst equation, and because several reactions proceed at once on a metal's surface, the value settles at an intermediate point called the mixed potential. The phenomenon where two different metals touch and one dissolves away is known in engineering as galvanic corrosion, and it's also a problem for ships and pipes. In dentistry, the current — and the resulting pain or change in taste — caused by different metals in the mouth is treated under the term "oral galvanism."
📖 For the derivation of the formula and further reading: Standard electrode potential (Wikipedia, Japanese) / Galvanic corrosion (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- Exactly where electricity stimulates the nerve. Whether it directly shakes the nerve endings in the pulp, or acts through the movement of fluid in the dentinal tubules, the precise pathway is still debated.
- Why the same filling causes pain in some people but not others. Factors such as how deep the filling sits, the composition of saliva, and how sensitive the nerve is all seem to play a role, but how much each one contributes, and for whom, hasn't been clearly sorted out.
- Whether weak currents in the mouth affect the body over the long term. Researchers are studying the relationship between metal dissolving from different fillings under a continuous weak current and overall health, but no conclusion has been reached.
In short, even this article describes things only "as currently understood." The broad mechanism behind the pain from foil is solid, but the finer details of what happens inside the nerve are left to future research.
Links to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Third-year science, "Chemical change and ions" (batteries) | Two metals plus moisture make a battery (Reason 1, Figure 1) |
| High school | Chemistry, "Batteries and electrolysis" / Biology, "Nerves and receiving stimuli" | Standard electrode potential differences, nerve signals (Check with a formula) |
| High school+ | Tooth structure (textbook sidebar material) | Dentinal tubules and the hydrodynamic theory |
| University | Electrochemistry, corrosion engineering, dentistry | The Nernst equation, mixed potential, oral galvanism |
| Research | Oral physiology, dental materials science | Pathway of electrical stimulation, individual differences, long-term effects |
| ― | Links to daily life | Handling foil, silver fillings and forks, consulting about tooth pain |
- 標準電極電位 (Standard electrode potential) - Wikipedia (Japanese)
- ガルバニック腐食 (Galvanic corrosion) - Wikipedia (Japanese)
- 象牙質 (Dentin) - Wikipedia (Japanese)
- The Chemical Society of Japan (日本化学会), ed., *Kagaku Binran: Kiso-hen* (Chemistry Handbook, Basic Edition), Maruzen Publishing (for standard electrode potential values)
- Brännström, M., a series of papers from the 1960s on the hydrodynamic theory of dentin sensitivity
※This article is a general-audience science explainer. The figures given are approximate values meant to aid understanding of the underlying mechanism. If tooth pain persists, or if you notice discomfort around a filling, please consult a dentist.