Xerox's Erasable Paper.

Xerox scientists have created the erasable paper: image and text printed on it disappears within a day, and the paper can be reused again and again (which saves trees!)

To develop erasable paper, researchers needed to identify ways to create temporary images. The "a-ha" moment came from developing compounds that change color when they absorb a certain wavelength of light but then will gradually disappear. In its present version, the paper self-erases in about 16-24 hours and can be used multiple times.

Link – via Tech Ticker