During the War, a mathematician, a chemist, and a mechanical engineer are captured by the enemy. They're thrown into three jail cells in a remote part of the enemy base, and quickly forgotten. A month later the jailer remembers that these three prisoners haven't been fed in a month, and goes to check on them.
In the first jail cell, he finds a rudimentary lever and wedge on the floor. The mechanical engineer had pried the cheap cell bars apart, and escaped handily.
In the second jail cell, he finds a puddle of acid: acid that the chemist had poured on the cell bars to free himself.
In the third jail cell, he finds the mathematician's rotting corpse. Next to the body, there's a piece of paper. He reads it. It says: "Assume that I have the key to the cell..."