Puzzle of the Week for 16 November 1998

A friend tells you that you can flip an ordinary penny 10 times and that it will land heads up 10 times in a row. To prove it, your friend gives you the penny, and following her instructions, you toss it 10 times. Sure enough, it lands heads up 10 times in a row. Afterwards, you toss it again, and it lands tails up about half the time. How is this possible?