A simple Jew from the infamous city of Chelm visits Warsaw. He meets the shamash of the shul and the shamash asks him a riddle: Who is my father's son, but he's not my brother? The simple Jew thinks and thinks, finally gives up and asks: Who is it? The answer is simple: "Me."

"That is great," he thought! When he returns to Chelm he gathers his friends and family and challenges them with the riddle. Who is my father's son, but isn't my brother? They give up. "It is the shamash of Warsaw!" he proclaims grinning at the brilliance.