A: Gravity is very important.
B: What is gravity?
A: It's the force that pulls everything down.
B: I don't understand.
A: If you pour water into a glass, the water goes down into the glass.
B: Of course it does.
A: Without gravity, the water would go up.
B: You're joking.
A: Without gravity, you would go up.
B: What do you mean?
A: You would float into the sky like a balloon.
B: That would be fun!