A scientist and an engineer were sharing a prison cell, both sentenced to be shot at dawn. Fortunately, they came up with a plan. As the physicist was led out to the firing squad, the engineer set fire to a small pile of straw on the window-ledge of their cell.
“Look!” the physicist yelled to his captors. “The prison is burning!” The firing squad dropped their weapons and ran to put out the conflagration. Furthermore, the Warden decided that the scientist deserved a pardon for saving the prison.
The next day the firing squad came for the engineer. As you might guess, the scientist was having a much harder time getting a large enough fire going outside the prison to draw attention. As it happens, it wasn’t until the engineer was staring down the rifle barrels that he finally saw enough smoke. So, as fast as he could, the engineer yelled “Fire!”