A physicist and a mathematician were asked to remove two nails, one of them punched all the way into the wall, the other just half-way. The physicist just pulled out the one that was half-way in and then, after toiling some time, managed to pull out the second one. The mathematician started with the one that was all the way in the wall, since it was more interesting. After some considerable time and effort, he managed to get it out. Then he looked at the other one, and uttering the words "This can be simplified to an already solved case" punched it all the way into the wall.