Yup, I remember a math teacher did that to us, bet us something that there were two people in the room that had the same birthday.. intuitively it seems like a bad bet but if you work out the numbers it isn't.
You can use this to take advantage of people too.
I'm a great market predictor. I send out 32,000 newsletters predicting the direction the TSE will take over the next week. Half I send out say up, half say down. Next week I send out 16,000 to the ones that I got right. Next week send out another 8,000. After six weeks I'll have 500 people falling over themselves to give me their money because I could predict the stock market for 6 weeks in a row.
