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How Does Drinking Water with a Straw Work?

Those bendy straws will never look the same

You do it every single time you order a drink when you go out to eat. Your waiter/waitress gives you a plastic tube covered in paper, you unwrap it, stick it in your drink and you slurp away. Ever wonder how you get that liquid to flow up through that thin plastic tube and into your mouth? It’s all simple physics really and this post is going to unravel the mystery behind straws.

The Atmosphere

Before you put your mouth on the straw, the fluid is exposed to atmospheric pressure, which is the pressure exerted by the air onto the surface of the fluid, say water, in your cup. The pressure on the inside of the straw is the same as the pressure on the water, which is why you don’t see any movement in your cup and is why the water level inside the straw is the same as the water level in the cup.

But if you were to place you finger on top of the straw and pull it out, you’ll notice that the water stays in the straw and this is because you have completely sealed the straw from the atmosphere, therefore the air cannot push on the liquid in the straw. It is the same idea when you place your mouth over the straw, as by doing that you seal off the inside of the straw from the outside atmosphere.

How does the liquid move up then?

When you begin to suck on the straw, you increase the volume in your mouth by bringing your tongue down and expanding your jaw. Doing this causes the pressure to decrease therefore creating a scenario where the pressure inside your mouth is lower than the outside atmospheric pressure pushing down on the liquid. The pressure in the straw also becomes reduced, thus causing the atmospheric pressure outside the straw, which is greater, to push down on the water and through the straw and into your mouth.