Fish

There was once a Polish guy who had a Jewish neighbour. He went to visit him and wanted to know why the Jews are so smart. "We eat a lot of fish," said the Jew. "Can I have some?" "It's gonna cost you $100 a piece." "If that's gonna make me smarter, I'm willing to try," said the Polish guy.

He ate a piece of it, then said the Jew, "You know, this is quite a lot of money for a simple piece of fish. I really don't believe I should have paid such a sum."

"You see? It already works..."