How a Straw Works
When water is in a glass it experiences a downward force (gravity) which keeps it in the glass. In order to drink the water through a straw there must be a force up that is stronger than the force of gravity downwards. That means that we must either pull the water up the straw or it must be pushed up the straw.
Before we see which of those possibilities is the correct one, let's make sure we understand the difference between pulling and pushing. Quite simply, pushing is when the force is applied behind the direction of motion and pulling is when the force is applied in front of the direction of motion. The diagram below may help to make that more clear.
So, in order to pull something you need to be able to apply a force from the front. There are only a few ways to do this. Magnets can apply such a force to each other, as can charged particles (when they are VERY close), but water is not magnetic and no charge is used to drink from a straw. So the only way left to pull something is to grab it from the front. However, in this case, this is impossible. You cannot grab water – it has no hooks or handles.
That means that you cannot pull the water up the straw. Really. Can't be done. Don't even try to argue your way out of it. When you drink from a straw, you are NOT pulling the water up the straw.
That leaves only one possibility. The water is pushed up the straw. The obvious question is “by what?” Let's take a look at what is going on in and around that straw. When the straw is just placed into the water the water fills the straw up to the height of the water outside the straw. In this situation, the air above the glass is pushing down on the water both in the straw and around the straw. Since the forces are the same (it's the same atmosphere) the forces are balanced, the water levels are balanced and nothing moves.
It is easiest to imagine this if you picture a see-saw at the bottom edge of the straw. Inside the straw (on that side of the see-saw) the water in the straw pushes down, as does the air in the straw. Outside the straw (on the other side of the see-saw) the water pushed down as does the air outside the straw. Since the height of the water is the same, the pressure from the water is the same in and out of the straw and the air pressure is the same.
When you drink from the straw, you reduce the pressure in the straw. (How you do that will be explained later in the unit.) Suddenly, the forces are unbalanced.
In this case the force down on the inside part of the see-saw is weaker than the push on the outside side of the see-saw. As a result of the unbalanced forces, the water (the see-saw) will move. In this case the water outside of the straw will be pushed down and the water in the straw will be pushed up.
When you drink from a straw, you are actually allowing the water to be pushed up the straw by the atmosphere.