I had a reality check about how much unconscious teaching and experimenting I do at the park yesterday when DS2.7 was playing trucks in the sandbox with a nice little 4 yo boy.

Boy: Look! The sand sticks together!
DS: Yes, that's because of surface tension.

Later, boy tests driving DS' truck down a slide:

Boy: The truck fell! Look! The truck fell!
DS: Gravity pulled it to the ground!
Me (to the boy): Try sliding it down on its wheels, then on its side, and compare the truck's speed in both situations. Which do you think will be faster?
Boy (after testing): Wheels make it travel faster, neat!
Me: Why do you think that is?
Boy: I don't know.
DS: The wheels roll, so there's less friction on the wheels.

What is to give light must endure burning.