We make a small hole in a plastic bottle, about a half of its height, and we fill it full of water, while we are covering the hole.
If we remove our finger from the hole, no water comes out, but if we remove the cap of the bottle, the water starts to come out. But why?
If the cap is put, the internal pressure on the hole is equal to the external pressure and that’s why the water doesn’t come out of the hole.
But, if we remove the cap, we allow that the air comes into the top of the bottles, breaking the previous balance. The internal pressure on the hole is greater than the external pressure, so the water comes out of the bottle.
by Irene and Rafael