A mathematician, a physicist, and an engineer are all found guilty of treason...

and are sentenced to beheading by guillotine.

On the day of the triple execution, they are brought to be beheaded. The priest blesses the execution, saying all is done in God's name, and the King orders the three executed.

The mathematician is to be killed first, and the executioner gives him two options: would he like a hood on or off, and would he like to be face up or face down.

So the mathematician says "Well, they all arrive at the equivalent state of beheading, so it's an arbitrary choice. I suppose I'll take no hood and face down, since it's the simplest solution."

So they lock him into the guillotine, the executioner drops the blade... and it stops inches from his neck.

The priest exclaims "It's a sign from God! This man is innocent! He must be let free!"

The King agrees, pardons the mathematician, and lets him go.

Next up to be killed is the physicist. He is given the same options: hood on or off; face up or face down.

So the physicist thinks, and says "Well, I've spent my whole life looking up at the stars and studying them... but it's broad daylight. I will take the hood to make it dark like night, and face up towards the Heavens."

So they lock him into the guillotine, the executioner drops the blade... and it stops inches from his neck.

The priest exclaims again "It's a sign from God! This man is innocent! He must be let free!"

The King agrees, pardons the physicist, and lets him go.

Finally, it's the engineer's turn to face the guillotine. He's given the same options: hood on or off; face up or down.

The engineer ponders, and says "I've spent my whole life studying machines and mechanical devices. If I am to be killed by the most efficient killing machine of all time, then I will look at its workings while I die! Hood off and face up!"

So they lock him into the guillotine, and the executioner is about to drop the blade, when the engineer exclaims: "I see the problem!"