This is a BMW X6 with heat-sensitive paint that reveals The Hulk when doused with hot water (previously: this car treated with the same style of paint). Pretty cool, right? Unfortunately, cars usually don't get doused with hot water that often. I guess pee would work too. Have I told you somebody's been repeatedly peeing on my car at night? I set up a video camera to catch them. It was a cat. Just kidding, it was me sleepwalking, but that's embarrassing so let's just say it's a cat. "But you just admitted it was you." IT WAS A STRAY.
Keep going for a video of the transformation in process.
Thanks to Krame, whose car's paint job changes over long periods of time. Yeah, that's called rust.