Favorite science fair project you did as a kid and why?

Can't remember any myself outside of a really simple one I did with my Dad at about age 6. We took three cups of water and dispersed each one in a different place with a different temperature — outside in Chicago winter, inside the house, and on a lit stovetop burner — and then put all three in the freezer and checked how long it took each one to freeze. Surprisingly to me at the time, the hottest water actually froze the quickest. Something to with vaporization or the so-called "Mpemba effect," although some quick research proves the question of why this happens remains open for debate. What about you? Baking soda volcanos = automatically disqualification.