Q: What has 4 wheels and flies?
A: A garbage truck!

Explanation:  When you hear “4 wheels and flies” you expect that the word “flies” means something that flies in the air like an airplane or perhaps Superman.  You do not expect that “flies” means the insect, fly (or rather the plural of fly: flies).

It’s true: a garbage truck has 4 wheels and lots of flies because flies like garbage.

And just for fun, here is a flying garbage truck: