What a great question and one I am sure that we have all wondered about while sitting in the bath! So here you go.. your skin is covered in natural oils which helps make it waterproof. If you wash your hands normally then the oil stays on, so your fingers stay the same. However when you sit in a soapy bath for a long time the oils will start to be broken down and come off your hands. This allows the top layer of your skin to become waterlogged and it starts to go wrinkly. Some Scientists believe that is may give us an evolutionary advantage as it is easier to pick up objects with wet, wrinkly fingers. However I recently watched an episode of the fantastic programme Operation Ouch (which is brilliant for teaching kids all about the human body - Picture of Dr Xand below!) and they did this experiment. They found out that it was no easier to pick up objects with wrinkly wet fingers than with normal wet fingers – so it may not help us, but we do know why it happens. Of course after you get out of a bath your fingers gradually return to normal and the natural oils are secreted onto your skin once again. I hope that explains it and that you keep asking fantastic science questions!!