Q: How do you know a wishing well works?
A: If your mother-in-law falls down it.
Q: How do you know a wishing well works?
A: If your mother-in-law falls down it.
This is a dark joke that plays on the stereotype of a bad relationship with a mother-in-law. It implies that the only way to know a wishing well works is if something bad happens to the mother-in-law.