"True randomness must be based on the inherent unpredictability of our universe. Mr. Haahr's iPod engraving is a rebuttal to Albert Einstein's famous objection to quantum mechanics: 'God does not play dice.' In fact, subatomic particles often act as if governed by a roll of dice. The decay of a radioactive nucleus, and the orbit of an electron around an atom, are processes that are inherently unpredictable.

Hence, when John Walker, founder of the software company Autodesk, decided 20 years ago to create his own source of random numbers in his computer laboratory in Western Switzerland, he bought some radioactive Krypton-85. The substance decays at a known rate, but the timing of the decay of individual atoms inside the Krypton-85 is entirely unpredictable. Mr. Walker monitors the decay, and distills that randomness into a random sequence of 0s and 1s that are published on his Web site, HotBits.

There are other Web sites aimed at serving up random numbers. Random.org comes up with numbers by monitoring atmospheric noise generated from a radio tuned to a frequency without a station. LavaRnd.org uses a digital camera with its lens cap on, mining the chaos within the distribution of light on the dark lens. These tools can produce enough random numbers for many applications. They're the philosophical descendants of a famous Rand Corp. publication from 1955 called 'A Million Random Digits,' which is 'probably our best seller,' Rand senior statistician John Adams said, laughing.

But these strategies, which must detect and analyze natural processes, serve up numbers too slowly for high-volume consumers of randomness. Banks, casinos and others generally must rely on computer algorithms, meaning they're on the hunt for programs that can best mimic true randomness. (Researchers also use random numbers for simulations, but unlike those using the numbers for security, researchers need a record of the numbers used so that they can redo the experiment to verify the findings.)

How would you know if your sequence of numbers is random? Just looking at them wouldn't help. 'People are notoriously bad at being a random number generator or recognizing something as random,' said Landon Curt Noll, one of the creators of LavaRnd.org. People tend to seek patterns and order where none exist -- perhaps even in a shuffled iPod playlist, where they might pay more attention when their favorite songs are playing, and thus assume that those songs are in heavier rotation."