Crazy Aaron's Hypercolor Thinking Putty is thermochromic - meaning it changes color with temperature! Now a touch from your hands will change the putty's into its "hot" color, and if you stick it in a fridge (or allow it to cool off), the "cool" color will return.
Now, what's extra neat about this is that you can microwave the putty and use that to measure the speed of light (you know the c from Einstein's E=mc²). Crazy Aaron even got within 5% of the actual value of c!
Links: Crazy Aaron's Puttyworld | Using Putty to Measure the Speed of Light - [You should know, Crazy Aaron advertises on Neatorama - you can see his banner on the right-hand side (go visit, people!). Still, ain't it cool?]