Sir Isaac Newton and the Cat Flap.
Sir Isaac loved his cat but it annoyed him when it kept nudging the door of his laboratory open and allowing unwanted sunshine into the lab while he was conducting his light experiments. He did not want to stop the cat from coming into the laboratory so he made a hole in the door and covered it with a piece of felt. He trained the cat to use the flap and all was well. Then, when the cat had kittens, Sir Isaac made a smaller flap for them not realising that they could use the existing flap!
Cat Flaps are extremely useful when bringing home a Squee (a mouse, vole, mole, shrew, etc.) for tea!