Everybody on the planet probably knows this simple trick. All you do is take a straw, submerge part of it inside of a liquid, cover the top hole of it with your finger, then take it out, and voila! The liquid stays inside of the straw rather than draining out, as gravity intended.
But how does it work?
It's actually pretty simple, but most people don't really think about it. If you just stop reading for a minute and really just think, you'll figure it out.
I didn't just make this post to tell you to think. This is for a grade, so I need to put at least some effort into it.
It's all a matter of pressure. By plugging the top of the straw, you isolate the air on the inside from the atmosphere. If the liquid were to start draining from the straw, that would increase the volume that the air would have to take up, without increasing the amount of air in the straw. If you were to turn it upside-down, the liquid won't move, it'll stay hovering in the straw, because if it were to start falling down, that would decrease the volume that the air would have to take up, without decreasing the amount of air in the straw.