This trick will make you look like the new Rain Man - or a great mindreader. You will be able to guess the age and birthday of a total stranger. There is some mathematics involved that is so easy that no one will figure out how you do it. There is no fake calculator or toxic force involved. This is a complete different trick.

You can do this totally impromptu. It only demands a stranger/spectator with their own phone (which you never touch).