Conversation with Jen earlier today:
R: I've just worked out why animals' rear legs are stronger than their front ones.
J: Why's that?
R: Because they get their strength from the buttocks, and there isn't room up front for another pair of buttocks.
R: Wouldn't it be weird if we had a second pair of buttocks under our armpits?
J: Well, toilets would look very different for a start.