I always find this interesting… and gross.
Below are pictures showing the inside of 3 computers, all showing similar areas of the processor fan.

This is what the processor fan of a new computer looks like:
Fan-New

Here is what you can expect one to look like in a computer that is around 5 years old:
Fan-5yrs

This picture is a computer also around 5 years old from a home where cigarettes are smoked. The fluffy stuff around the inside of the fan is about a half inch thick! The same substance is equally caked on every other part in this computer. Breath deep my friends!
Fan-5yrs_Smoke