Why Can You Drink Through a Straw?
― It's the Atmosphere Doing the Sucking, Not You
When you drink through a straw, it feels like you're "sucking the liquid up" with your mouth. But physically speaking, your mouth's strength isn't what's lifting the liquid. The real culprit is the invisible weight of the air that constantly surrounds us — atmospheric pressure.
You put a straw in a drink and give it a firm suck, and the liquid rises into your mouth. From that feeling, it's natural to imagine "my mouth is pulling the liquid up."
But there's no "hand" inside your mouth that can grab and pull the liquid. All your mouth can actually do is expand your lungs and push the air inside your mouth and the straw out.
So why does the liquid rise at all?
When you suck on a straw, the air pressure inside the straw drops. You aren't pulling the liquid.
The surface of the drink in the cup is always being pressed down by atmospheric pressure. As the pressure inside the straw drops, this atmospheric pressure pushes the liquid up.
Not "pulled" but "pushed up" — let's walk through this shift in perspective step by step.
"Sucking" really means "lowering the pressure"
When you put a straw in your mouth and suck, you're expanding your lungs and moving the air inside your mouth and the straw into your body. As the air decreases, the pressure inside the straw drops below the surrounding atmospheric pressure.
This is the key point. Your mouth never touches the liquid itself, not even once. All your mouth is controlling is the "air pressure" inside the straw — nothing more.
What pushes it up is the outside atmospheric pressure
The surface of the liquid in the cup is constantly under pressure from the weight of the atmosphere — something we normally never notice. As long as the liquid feels this same atmospheric pressure both inside and outside the straw, the forces stay balanced and the liquid doesn't move.
But once the pressure inside the straw alone drops, that balance breaks. The atmospheric pressure pressing on the liquid surface outside the straw becomes relatively stronger, so that pressure pushes the liquid up inside the straw. This is the real mechanism behind drinking through a straw.
An invisible giant called atmospheric pressure is doing the pushing for you.
There's actually a limit to how high a straw can lift a drink
This "pushed up by atmospheric pressure" mechanism has a theoretical limit. Even if you could completely remove all the air from inside a straw, creating a perfect vacuum (zero pressure), there would still be a maximum height that atmospheric pressure could push the liquid to. That's because the size of atmospheric pressure — the "pushing force" itself — is a fixed value.
In the 17th century, the Italian scientist Torricelli noticed that when pumping water up, it simply wouldn't rise above a certain height, and through this he uncovered this limit. We'll work out exactly how high that is with numbers in the collapsible section below.
This limiting height is said to be vastly larger than the length of an ordinary straw. In everyday use, you never need to think about this limit at all. That said, in situations like pumping water up to very great heights, this limit can become a real design constraint.
Something you can check for yourself
- Fill a cup with water and push a straw deep into it
- Seal the top of the straw firmly with your finger and lift the straw out of the cup like that
- Check that while your finger keeps it sealed, the water inside the straw stays put without spilling
- Also check that the moment you lift your finger, the water pours out of the straw
While your finger seals it, the air pressure below the water inside the straw balances the outside atmospheric pressure, holding the water up. When you lift your finger, that balance breaks, atmospheric pressure pushes air in from below, and the water falls.
Summary
You can drink through a straw not because your mouth is directly pulling the liquid up. By drawing the air out of the straw with your mouth, the pressure inside the straw drops, the outside atmospheric pressure becomes relatively stronger, and that pushes the liquid up. This "atmosphere pushes it up" mechanism does have a theoretical height limit, but an ordinary straw has more than enough room to spare, so it never becomes a problem.
When you drink through a straw, you aren't the one supplying the force.
All you're doing is opening up a tiny "gap" for the enormous force of the atmosphere to act through.
For an example of the same atmospheric pressure working in the opposite direction, see also the story of why an upturned plastic bottle glugs instead of pouring smoothly. There, the atmosphere is what holds the water back.
Want to know more? ― terms, numbers, and links to the textbookWe've labeled which level each topic belongs to, from middle-school science to open research questions
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics"
- HS+Covered in high-school "Physics," or treated as advanced/sidebar material in textbooks
- Univ.Not taught in high school — a specialized university subject (fluid dynamics)
- ResearchNot even settled fact at university level — something researchers are actively investigating
MSTerms: the vocabulary of straws
- Air pressure (atmospheric pressure): the pressure exerted on surroundings by the weight of air.
- Vacuum: a space with almost no air (gas) present.
- Pressure difference: the difference in pressure between one location and another.
HSChecking the numbers: how high can atmospheric pressure push water?
Let's calculate the theoretical height limit that atmospheric pressure could push water to.
Limit height = Atmospheric pressure ÷ (Density of water × Gravitational acceleration)
| Atmospheric pressure | roughly 101325 Pa (pascals) |
| Density of water | 1000 kg/m³ |
| Gravitational acceleration | roughly 9.8 m/s² |
This equation finds the height at which the pressure from the weight of the raised water column exactly balances atmospheric pressure.
| Density of water × Gravitational acceleration | 1000 × 9.8 = 9800 |
| Atmospheric pressure ÷ (density×gravity) | 101325 ÷ 9800 ≈ 10.34 |
| Limit height | about 10.34 m |
In theory, the limit for how high atmospheric pressure alone could push water works out to about 10.34 meters. That's roughly the height of a 3- to 4-story building.
Let's compare that with the length of an ordinary straw (a typical figure being 20cm, or 0.2m).
| How many straw-lengths is the limit height? | 10.34 ÷ 0.2 ≈ 51.7 |
| How many straw-lengths is the limit height? | about 51.7 times |
An ordinary straw's length works out to be only about 1/50th of this theoretical limit. That's exactly why we can enjoy a drink through a straw without ever thinking about this limit.
※ Atmospheric pressure varies somewhat with location and weather; the figures here are representative estimates.
HS+Torricelli's vacuum and the mercury experiment
In 1643, Torricelli ran an experiment in which he filled a long tube, closed at one end, with mercury and stood it upside down with its open end in a container of mercury. The mercury in the tube dropped and stopped at a certain height (about 76cm in today's units), leaving an empty space (a vacuum) at the top of the tube. Because mercury is far denser than water, the same atmospheric pressure can only push it up to a much lower height than water. This experiment is famous as the first to demonstrate the concept of atmospheric pressure quantitatively.
Univ.How this differs from the "siphon principle" in fluid dynamics
A phenomenon similar to the straw is the siphon, where a tube is used to move liquid from a higher place to a lower one. The siphon principle is also related to atmospheric pressure, but the pressure difference from the liquid's own weight is considered the main driver of the flow, so the forces at work differ from drinking through a straw. In fluid dynamics, the motion of a liquid is thought to be determined by a combination of factors, including pressure, gravity, and viscosity.
ResearchWhat's still unclear
- Research into microfluidics — technology that uses tiny channels (microchannels) to precisely handle very small volumes of liquid — is progressing in fields like medical diagnostics and chemical analysis. Researchers are studying how to control the same "pressure difference moves liquid" principle used by a straw, but at a much smaller scale and with much greater precision.
- As for exactly how the tongues of nectar-feeding creatures like hummingbirds actually move nectar, this was long thought to be explained by "capillary action" alone, but recent research suggests more complex fluid-dynamic mechanisms may be involved that simple capillary action can't fully explain, and the research continues.
The physics of atmospheric pressure behind a single straw connects all the way to the mechanisms of living creatures and to cutting-edge medical technology.
Links to the textbook (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Science: gases and pressure | Basic terms: air pressure, vacuum, pressure difference |
| HS | Physics Basics: pressure and force balance | Calculating the limit height from atmospheric pressure |
| HS+ | Physics: history of gases and pressure | Torricelli's vacuum experiment |
| Univ. | Fluid dynamics | Difference from the siphon principle |
| Research | Microfluidics / biological fluid dynamics (ongoing research) | Microchannel control technology, research into biological suction mechanisms |
- High-school "Physics Basics" textbook explanations of atmospheric pressure and Pascal's principle.
- Historical-physics reference materials describing Torricelli's vacuum experiment.
- Fluid-dynamics textbook explanations of the siphon principle.
- Research review of liquid-handling technologies using microchannels in the field of microfluidics.
- Research review on hummingbird nectar-feeding mechanisms in the field of biological fluid dynamics.
※ Values such as atmospheric pressure and density are representative estimates; actual values vary with environmental conditions.
※This article is a general-audience science explainer. For guidance on the safe way to drink beverages (such as precautions against choking), please consult medical resources as needed.