The priest put his head in the slot and the executioner pulled the lever; the guillotine blade came speeding down the track and stopped just a few inches above the priest neck. The priest proclaimed that God had intervened and saved him from execution; the executioner had to agree and let him go.
The mathematician had a disbelieving, puzzled, look on his face.
Next the Rabbi put his head in the slot, the executioner pulled the lever and the blade came speeding down the track and stopped a few inches above the Rabbi's neck. The executioner agreed that God had intervened again and saved the Rabbi also.
The Mathematician, more troubled than ever, put his head in the slot and turned to look upward and he noticed something that made him smile.
Before the executioner could pull the lever, the mathematician said "Hold on there a minute, I see what the problem is! The track has a small pebble blocking the path of the blade". He removed the pebble and announced, "There, it should work just fine now!"