How many flies are in my apartment?
In my statistics class we were taught a technique for estimating the number of flies in a barn. You capture a fixed size sample of flies, e.g. 100, and give them each a little white dot on the back, then release them all back in the barn. Leave them to mingle for a while, then capture another fixed number of flies, e.g. 100. If 5 of them have a white dot on them, then you estimate that the 100 flies you captured originally and marked make up 5% of the total population, meaning your estimate of the total population must be 2000 flies.