What has four wheels and flies?

Q: What has four wheels and flies?
A: A garbage truck!

This is both a riddle and a pun. The humor in this joke lies in the fact that the person says "four wheels" and "flies" close together. Having wheels brings to the hearers mind the idea of locomotion. Hearing the word "flies" while thinking of locomotion makes the hearer hear flies as "the ability to fly" instead of "flying insects". When the answer is revealed, the hearer first is confused because garbage trucks don't fly, and then realizes that garbage trucks are reported to have flies.

As a side note, I have never seen a fly near a garbage truck.