As a kid, my favourite Hot Wheels toy cars were the ones that changed colour in cold or hot water. Lots of fun in the bathtub! But eventually we all grow up and accept the fact that real cars don't change colours (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.