Pass it down the pipe

This one is interesting and can be made from empty cardboard boxes. I used empty toothpaste boxes to make this and was thoroughly enjoyed by my child.

Materials needed: Empty toothpaste boxes - (at least 2)
                              Small ball that fits into the toothpaste box
                              Wall adhesive (double sided tape)

Stick the empty toothpaste boxes in various inclinations such that when ball is dropped into the top box, it flows down and comes out of the bottom one.

I stuck one vertically and the other inclined.

Dropping a tiny ball into the top opening.

Observing what is really happening at the juncture. How can an item that is dropped into the top opening come out of the bottom one?

Observing the bottom opening.

1. 2. 3. Repeat. You can sit back and enjoy your child's reaction. Of course, do not forget to add a background voice on what is happening and how or why that item is behaving so.