You may think you are “sucking up” liquid into your mouth when you are using a straw in your drink, but this is where our common sense notion of the world is wrong.
How do straws work?
In reality, when you expand your lungs, you create a low pressure zone in your mouth. This means that you have a pressure in your mouth (and therefore in the straw) lower than atmospheric pressure. At 14.7 pounds per square inch, the atmosphere pushes down on us quite a bit.
In fact, if you placed a straw in a glass of water like the diagram above, you could fill a straw that is 30 feet (~10 meters) high without any liquid flowing into the glass. This is because the weight of water in the straw is matched by the atmospheric pressure pushing down on the water in the glass. This illustrates just how much the atmosphere pushes down on us.
At sea level, the air pressure is enough to support a column of water about thirty feet high. This means that even if you could suck all the air out of a forty-foot straw, the water wouldn’t rise more than thirty feet.
When you create an area of lower pressure in your mouth (and the straw), there is no longer a balanced pressure. The higher pressure outside of your mouth then forces the water up the straw and into your mouth to restore equilibrium. So, the weight of the atmosphere pushes liquid through the straw and into your mouth, no sucking involved. In science, the concept of suction is not an attractive effect: matter is not sucked into the ares of low pressure. Instead, matter from a high pressure area is pushed into a low pressure area. Suction is therefore a pushing force and not an attractive one.
Would a straw work in space?
Because a straw depends on the surrounding pressure of the atmosphere to push matter into our mouths, it follows that a straw could not work in space. Because empty space is (nearly) a vacuum, there is nothing to force the water into your mouth. However, being that you will probably will never find yourself floating in empty space, a straw would still work on a space shuttle (which you could possibly find yourself in…maybe) because the cabin, like an airplane’s, is pressurized.