At some point, the warden realizes that the three men haven't been fed in a while. He accompanies an officer to check up on them.
The warden and officer arrive at the first cell that contained the engineer. To their astonishment, the cell was empty and the wall had a hole in it.
"How is that possible?" said the officer. "That wall was solid concrete!"
The warden quietly inspected the cell. After a few minutes, he exited and said, "He seems to have built a pick out of the eating utensils we gave him and used it to make the hole."
The warden and officer continued to the next cell that contained the physicist. However, he too was gone and once again there was a hole in the wall.
And of course, the warden inspected the cell and returned after a few minutes.
The warden declared, "According to the papers on his bed, he very carefully calculated the weakest point on the wall and repeatedly hit it with a rock until it broke open."
Finally, they arrived at the last cell that contained the mathematician. Unfortunately, he lay dead on the cell floor from starvation. The officer sighed.
"After the other two, I would have expected he would have also escaped. He also has some papers on his bed." The warden entered the cell and picked up the papers.
Then he shook his head and chuckled.
"It appears," the warden said, "that he spent several days writing a very detailed proof that it was possible to break the wall.
PS: My math professor told me this joke last year, I was reminded of it by yesterday's top post.
EDIT: Holy shit, my #1 post! Thanks for the upboats. Also send nudes.