A rewritable paper(upto 20 times!) ,which uses light as an ink.

How many times you use a paper? Just once. Next time we use it as a paper aeroplane. But now you won't. Scientists have discovered a paper which can be used upto 20 times. News didn't end here, you don't even need ink to write on it. Imagine how many tree would be saved every year.
There is a special dye embedded in the paper that makes it printable and rewritable. As movement of electrons occurs due to chemical reaction, the dye goes from dark to clear and then back. (Electrons are the subatomic particles that orbit in the outer regions of an atom.) The color change chemical undergo redox reaction. Redox is short for reduction and oxidation.

Due to oxidation of dye in new paper, it appears blue, red or green.(What color depends on which dye is in the paper.) When the dye on some parts is reduced, color on those areas disappears. It is due to the control of these two reaction which makes it possible to print,erase and reuse  it. This technology can also be used in conventional paper, glass given that it contains redox dyes and other chemical compund.