Three men are in a hot-air balloon. Soon, they find themselves lost in a canyon somewhere. We can call for help in this canyon and the echo will carry our voices far. A doctor, a lawyer and a mathematician were discussing the relative merits of having a wife or a mistress. If you have a wife and want a divorce, it causes all sorts of legal problems.
Algebraists do it in groups.
No cat has eight tails. A cat has one tail more than no cat. Therefore, a cat has nine tails.