Inform your target that new medical research has found a genetic link between the size of your hands, and the probability that you will develop cancer in later life. The details of the research are obviously very complicated, but it boils down to a simple rule of thumb; if your hand is bigger than your face, then you're very likely to develop cancer. The immediate instinct is for your victim to immediately check by placing their hand over their face, allowing you to slap their palm hard into their face. This is actually very painful, and runs the risk of hitting the secret Kung-Fu instant death spot, which shoots the nose backwards into the brain.