Magic Beer Bottle

You guys beat me to it. Look beside 99 90 81 etc all the labels are the same. No matter what number you originally start with, it will end up being a multiple of nine.

Kind of like the webpage that has you pick a card,and then is able to "guess" which card you picked. Took me awhile to figure this one out...

http://urbanlegends.about.com/library/bl_card_trick1.htm