A friend posted a link to The Magic Gopher. He's a physic and can read your mind. So do you beleive it?
It took me a little while to figure it out. I started from 10 and started seeing that the answer was always 9. That was the key. Each ten set next was a multiple of 9. And when you look at the chart the multiples of 9 have the same symbol. Walla, he knows what you are thinking. It's one of those classic games that people like to wow you with, but there is nothing really to it.