Fine Structure

Physics Buzz has a great workthrough of a Fermi problem - that is, a question that can be solved generally by making estimations and guesses for parameters.

The question was about how long it would take to deplete the oxygen to uncomfortable levels in a classroom with a certain number of students. Of course, classrooms aren't perfectly sealed and the size of the rooms vary wildly, but it's good to know that it is possible to do within a reasonable amount of time... you'll just have to visit the link to know exactly how long that is.