Cop 1 and Cop 2 are one mile apart. Cop 1 sees a car go the speed limit at 40 mph. Cop 2 sees the same thing. However, they radio each other and realize that it went from Cop 1's location to Cop 2's location in 30 seconds. The mean speed had to be 120 mph, so even though they didn't see it, they know he did it.