Stories for Powerful Ideas

I often reflect back on a memorable story that my professor told during a course on experimental psychology. It went like this:

A researcher was studying fleas by doing some experimentation on them. He would pull a leg off the flea and say "jump flea, jump!" and the flea would jump. He then would pull another leg off the flea and say "jump flea, jump!" and the flea would jump. Finally, after pulling off all the legs off the flea, the researcher said "jump flea, jump!" But the flea did not jump. The researcher concluded that when you pull all the legs off a flea, the flea goes deaf.

The moral of the story is that correlation does not imply causation.

Do others have memorable stories that they like that encapsulate powerful ideas?