A kid comes home from school and asks his dad what the difference is between theory and reality. His dad tells him to go ask his mum if she'd have sex with the plumber for a million pounds, he comes back and says she said she would. Good, now go ask your sister the same, the kid comes back and replies that his sister said she would too. Father replies, so in theory son, we're sitting on a couple of million quid, in reality it means your mother and sister are a pair of slappers.