The Missing Digit Trick

Ask a friend to secretly write down ANY number (at least four digits long). e.g. 78341

Ask the friend to add up the digits... e.g. 7+8+3+4+1 = 23 ... and then subtract the answer from the first number. e.g. 78341 - 23 = 78318

Your friend then crosses out ONE digit from the answer. (It can be any digit except a zero) e.g. 7x318

Your friend then reads out what digits are left .e.g. 7-3-1-8 Even though you haven't seen any numbers, you can say what the missing digit is! EIGHT

THE SECRET This great trick relies on the power of 9. How? http://mathema-tricks.blogspot.com/