As a kid, my favorite Hot Wheels toy cars were the ones that changed color in cold or hot water. Tons of fun in the bathtub! But eventually, we all grow up and accept the fact that real cars don't change colors (or jump through waterfalls). Or, you could hang on to those childhood dreams and paint your real car in thermochromic paint—like these crazy gearheads from the UK. Now to find a bathtub it'll fit in.