Are you old enough to remember mood rings – the color-changing plastic rings that supposedly could tell your mood by the temperature of your hand? RF Laserworks is offering a Heat Sensitive iPhone 4 Backing that sounds like more fun than those rings. At only 0.005″ thick, you’ll hardly know anything is on your phone. It’s thin enough to work with bumpers, too. It changes temperatures in the range of 90-95 degrees. As it is exposed to the heat from your hands or your pocket, it will change from black (cold), through red, yellow, green, then deep blue (hot). It will even reveal the outlines of inner components as they heat up from playing music, making a phone call, or surfing. The Heat Sensitive iPhone 4 Backing is $11.99, and you can order it with or without a cutout for the Apple logo.