Dr Chris and Dr Xand reveal the surprising secret behind the speed of your nerves, Dr Xand finds out what happens if you don't wash your hands, and the twins head out onto the street to test out a mind-bending trick that reveals how you can make your favourite cake look bigger than it is. Meanwhile, the ouch-mobile is opening its doors for more medical mysteries, and in accident and emergency one patient has a sweet stuck up their nose and another has an ear that is swelling fast.