Joke: How many 12 inch fish are in a foot?
Answer: Feet don't have fish in them so there are zero fish in a foot.
Joke: If 40 birds are sitting in a tree and ten of them are chirping, 15 of them are quiet and 15 of them are standing on one leg then, how many of them are flying?
Answer: All forty birds are sitting in a tree so none of them are flying.