As retold by Bruce Lowitt

 A Jew and a gentile are traveling on a train in old Russia when suddenly the gentile asks, "Why are you Jews so smart?"

"It's because of all the herring we eat."

Time passes, then  the Jew takes out some herring and starts to eat it.

"How much do you have?" the gentile asks.

"A dozen pieces".

"And how much do you want for a piece?"

"Twenty kopeks."

"That's a lot of money," the gentile says but, after a moment, he pulls  twenty kopeks from his pocket and exchanges them for a piece of herring.

After one bite, the gentile says, "This tastes like everyday herring. I could have bought a piece like this back in Moscow for a kopek or two."

"See?" the Jew replies. "One bite and already you're getting smarter."