Ball, wagon and father's answer?
When I was still pretty young--I don't know how old exactly--I had a ball in a wagon I was pulling, and I noticed something, so I ran up to my father to say that "When I pull the wagon, the ball runs to the back, and when I am running with the wagon and stop, the ball runs to the front. Why?"
How would you answer?
He said, "That, nobody knows." He said, "It's very general, though, it happens all the time to anything; anything that is moving tends to keep moving; anything standing still tries to maintain that condition. If you look close you will see the ball does not run to the back of the wagon where you start from standing still. It moves forward a bit too, but not as fast as the wagon. The back of the wagon catches up with the ball, which has trouble getting started moving. It's called inertia, that principle." I did run back to check, and sure enough, the ball didn't go backwards. He put the difference between what we know and what we call it very distinctly.
[...]
Feynman was a uber-master at explaining things. His little book, [link|http://www.amazon.com/QED-Strange-Princeton-Science-Library/dp/0691125759/sr=8-1/qid=1161131126/ref=pd_bbs_sr_1/002-4461508-0509612?ie=UTF8|QED], is beautiful.
Cheers,
Scott.