How Do We Drink Using a Straw?

         Something that many of us use on a daily basis is a straw, whether it be drinking soda or
Starbucks’ pumpkin spice latte. Now, if I were to ask you how we drink fluids through a straw and
the only restriction is that you aren't allowed to use the word “suck”, could you do it? Fear not,
because the answer is pretty simple; it all relates to air pressure.

 

         There is atmospheric pressure pushing against all objects all the time. When you pull the air out
of the straw by making a seal with your mouth, air will diffuse from inside the straw into your lungs
(air moves from PHIGH to PLOW). As a result, there is now less pressure inside the straw than outside
the straw. This means that when atmospheric pressure pushes down against the fluid outside of the
straw, it forces the non-compressible liquid to move up the straw and into your mouth. 

                               

Now, just as a followup question, what would happen if you sealed your cup completely so that no
air would be able to enter? Well, the absence of air would mean that the atmosphere won’t exert
pressure onto the surface of the liquid, and sucking on the straw would then be meaningless.