Your science teacher made a statement about CDs (Compact Discs) that you are not sure you believe. Your teacher said that your music CDs don't really hold any music on them. "They only have numbers recorded on them," said your teacher. How is that possible?

Your teacher is right in that sound is actually recorded onto the CDs as special numbers - a digital code. This digital code can't be seen as numbers. The code is pressed onto the CD as bumps on a long spiral track almost five kilometers (3 miles) long. These features look like "pits" on the opposite side of the CD. These pits, bumps, and grooves are an average of 0.5 microns wide (with a minimum of 0.8 microns wide). You need a million microns (a billion nanometers) to equal the length of a meter stick.

A small laser beam shines onto the bumps as the CD turns. The light is reflected back to a receiver that records how the laser light bounces back. This lets the CD player turn the reflected light back into the original code. This means you can hear the original code as music. How cool is this kind of technology?

Digital codes are used with many technologies. E-mail needs these kinds of code numbers. Space probes communicate with NASA using digital code. Bar codes are read as digital code in computer systems. Digital communications with cell phones need digital codes. Weather radios also tune in to specific signals using these codes.

Sometimes, the laser can misread a bump as it passes over it. The CD may be scratched or it was not recorded correctly. The digital codes include special error-correcting codes. This means you never miss a beat of your favorite song.

There are many types of compact discs. They can be recorded as CD-Rs. They can be recorded on and re-recorded (rewritten on) as you would do with a floppy disk. This is called a CD-RW. Another format is the CD-ROM. The technology for recording on these disks is different from other CDs. These CDs have a dye layer that the CD writer can darken or leave clear. The clear and dark spots are the digital code.

CD-ROM stands for Compact Disc - Read Only Memory. This disk is like a "super" floppy disk that can hold lots of information. One CD-ROM can hold the same amount of data as 500 floppy disks. Information is permanently recorded onto it. Computer games and other programs are considered to be CD-ROMs.

A recent step in CD technology is called the Mini Disc. It is half the size of a regular CD and can hold the same amount of information.

Some owners of CDs have wondered if their CDs can "wear out" from playing them over and over or "lose" their recording. CDs were first sold to the public in 1982. These CDs still play well and sound fine. Current CDs are expected to last between 70 to 200 years. Of course, you can make sure your CDs last a long time by taking care of them.

Your older relatives will probably always remind you about what music was recorded onto in the past. Most recently, audiocassette tapes were the main source for music. Before that, vinyl records were used (more commonly called "45s" and "albums"). These relatives may actually own a sample of this "ancient history" for you to see - and hear. Some people still believe that recordings on records sound better than CDs. One unusual difference between CDs and records is how the music is "read" as it plays. A record uses a needle that touches and moves across the record from the outside to the middle. A CD uses a laser that does not touch the CD and moves from the center to the outside.

It may not be many more years before a completely new technology is invented and introduced to the public for music recording. In the meantime, there is no doubt you will continue to enjoy listening to your favorite music on CDs and playing your favorite computer games on CD-ROMs.