A two-hole stopper is a bottle cap with two holes in it. We have two bottles, each with a two-hole stopper. One bottle is full of pink-colored water; the other is empty. There is tubing, inserted into one of the stopper-holes, going from one bottle to a stopper-hole in the other bottle.

Students may take turns on this project.

Lift the full bottle up as far as you can above the empty bottle. Keeping a finger over one hole of the full bottle, turn it upside down. Of course, the water runs through the tubing, into the empty bottle. As soon as it starts flowing, put the full bottle down beside the empty bottle, and watch what happens.

What is it that keeps the fluid flowing even after the full bottle has been placed beside the empty bottle, rather than above it?

Why is it that the liquid stops flowing as soon as each bottle has an equal amount in it?

Gravity makes this happen. The level of liquid in the empty bottle is lower than the level in the full bottle, until the empty bottle has as much in it as the full bottle. This
action is called a siphon (pronounced "SIGH-fun").