How do CD-RWs or rewritable CDs work?

The way a Cd works is by storing data in a digital format (0, 1), in a cd this is archived by creating thousands of bumps on the surface of a cd (the space left behing between bumps will look like pits)The way this is done is by making the surface of the CD a mirror, and the bumps disrupt the mirror's perfect surface thus making it less reflective.

The laser that reads the CD can detect the difference between a perfect mirror and an imperfect part caused by a bump because of the difference in reflection that is getting back. By interpreting the perfect mirror as a "1" and the bump or blurry part as a"0" digital information is store on a CD. The bumps on a CD are molded into the plastic when it is manufactured, so they are permanent.