How hard is it to pour water into a bottle? We'll see if we can make it difficult.

Carefully push a funnel through a one-hole stopper, and place the stopper tightly into a bottle. Slowly pour water into the funnel and observe what happens.

Not as easy as we expected, was it? Why?

Now, push a funnel through one hole of a two-hole stopper in another bottle. Pour water into the funnel.

Went right in, didn't it! Why?

Do we think that there was air in the bottles before we poured in the water? Of course, but so what? It was not easy for the air to move over and make room for the water.
When we poured water through one hole of a two-hole stopper, the air could escape through the other hole of the stopper. That made it easier for the water.

Have you ever watched mom open a milk can? Did you notice that she put two holes in the top of the can? Next time she opens a can of milk, ask if you can try it with only one hole in the can. It doesn't work very well. It might be that in order for the milk to pour easily, there has to be a way for air to get inside the can to push out the milk. Maybe we'll find out for sure.

For a variation of this activity, insert a U-tube or piece of plastic tubing in the second hole of the two-hole stopper. Fit a balloon over the end of the tube and tie it securely. Slowly pour water into the funnel until the bottle is filled.

What happens to the balloon? How is the balloon inflated?

Remove the balloon, empty the bottle, and hold a finger over the end of the tube. Fill the funnel with water again. Move the finger to open and close the tube, and observe what happens.

You will begin to realize that two materials cannot occupy the same space at the same time.