A story about Igor Tamm, the father of tokamak method of controlled thermonuclear fusion:

During the Russian revolution, the mathematical physicist Igor Tamm was seized by anti-communist vigilantes at a village near Odessa where he had gone to barter for food. They suspected he was an anti-Ukranian communist agitator and dragged him off to their leader.

Asked what he did for a living he said that he was a mathematician. The sceptical gang-leader began to finger the bullets and grenades slung around his neck. “All right”, he said, “calculate the error when the Taylor series approximation of a function is truncated after n terms. Do this and you will go free; fail and you will be shot”. Tamm slowly calculated the answer in the dust with his quivering finger. When he had finished the bandit cast his eye over the answer and waved him on his way.