D is for D-Ram. A low-achieving sheep? No, an essential element of any modern PC, and latterly the obscure object of criminal desire: police estimates suggest that last year pounds 200m worth of D-Rams were stolen in the UK.

Why? Well, D-Ram chips are more commonly known as "memory chips" - the acronym stands for Dynamic Random Access Memory. The chips store bits of data in their pure, digital form as 0s and 1s. Each bit is stored in a configuration of transistors known as a "flip-flop". D-Rams are where data from the programs you are running are generally stored, as it is thousands of times faster to access data on a chip than on a hard or floppy disk.

Improved production and miniaturisation techniques have cut the "price per bit" of D-Ram dramatically, as computers have become more widely used. In 1980, 1 megabyte of D-Ram would cost about $2,500; in 1995, it was $25. However, back in 1980, you would be quite happy if your computer had 256 kilobytes of Ram; nowadays, people think you are a wimp if you have less than 8Mb. In real terms we are paying less, but it does seem like a Red Queen's Race - running as fast as we can to stay stationary.

D-Ram chips, meanwhile, have become popular among thieves. They are easy to steal (open the computer and there they are), easy to conceal, legal to own, cannot be detected by sniffer dogs, and are in great demand both at home and abroad. For that reason, the black-market price per kilogram beats that of heroin. In the wake of the thefts have come various devices to make D-Ram chips burglar-proof, and insurance premiums for companies with lots of D-Ram-laden PCs have shot up.

So in fact, despite the manufacturers' best efforts in 20 years, the cost of filling your computer with memory has not fallen so much after all.