Is your refrigerator running?
Q: Is your refrigerator "running"?
Q: Then you had better go catch it!
This joke is often used as a prank call (back before you knew who was calling you prank calls were a way that kids would amuse themselves by calling people and telling jokes like the aforementioned joke).
This is a pun because the verb running can both mean 'properly functioning', and 'moving quickly by foot'. The person hearing the joke is supposed to assume that the person telling the joke is using the first definition, where the person telling the joke is actually using the second.
Additionally this is an absurd joke, because refrigerators can not ambulate in any manner (unlike washing machines, which can be said to walk at times during their spin cycle), so they couldn't be said to 'run' at all.