Herring

A crowded compartment in a pre-war train in Poland.
A Jew takes a herring out of his bag and starts eating it.
A passenger sitting opposite asks him:" Why do you Jews always eat some kind of fish?"
"It's because fish contain a lot of phosphorus, which is good for the brain.
That's why we Jews are so smart".
"Wouldn't you sell me one of your herrings?", asks the passenger.
"I can't sell a whole herring. But for one złoty you can get a herring head".
The passenger pulls out one złoty and gets a herring head.
But after eating it, something doesn't seem right to him:
"Wait a minute. You can buy a whole herring for one złoty, and I paid one złoty just for the head!"
The Jew answers: "You see. It's working!"
Passenger: "Indeed! Then give me two more heads!".