Ok, I confess I'm a bit of a science geek and have a big warm spot for science education. It doesn't take much to convince me to buy anything that can be justified as teaching the kids about science.
I recently discovered the PowerMagenetStore. Magnets are, of coure, one of the coolest things to play with and this place sells N38 Neodymium rare earth magnets. I ordered 18 of them for £13.25 and they quickly arrived in the post.
They are incredibly strong. A 1/2" cube easily holds up Jenny's wok (not that she appreciated it). Let two 1/2" cubes stick to each other and it takes a major effort to seperate them. These magnets can crush fingers and pinch skin very badly. I've let the kids play with them a little but they have to be strictly supervised. These magnets can easily kill credit cards, monitors, watches and other devices.