How do CDs, DVDs and hard discs store information?

We put this to Chris Bishop, Microsoft Research

That's a good question. CDs and DVDs work in slightly different ways but they have something in common, which is that like all computer storage systems they use binary. All the information's expressed in terms of strings of 1s and 0s or 'on' and 'off.' In a hard disc that's represented magnetically. Each bit is represented by a tiny magnet built into the surface of the hard disc and if it's facing north-up it's a 1 and if it's south-up it's a 0. The head that reads this information can also flip the magnets and so it can write information to the disc. The CD is rather similar. It also has these 1s and 0s but they're represented rather differently. They're represented by little pits. When a disc is written a laser burns little pits into the surface of the disc and another laser can read back those points. If there's a pit there might be a 0 and if there's no pit it might be a 1.