If a whole number is evenly divisible by 9, then adding up its digits will give you a number evenly divisible by 9, and you can repeat that process until you end up with 9 itself.  For instance, let’s say you have the number 5747106501.  If you add up those digits, you get 36, and if you add up those digits, 3+6 = 9.  Divide 5747106501 by 9 to check, and you get 638567389.  Works every time.

If you think that kind of thing is cool, you’ll like this video.  If you think that’s stupid and pointless, you might like the video anyway.