A random number is a number generated by a process that is fundamentally non-deterministic and unpredictable. Computer-generated "random numbers," which are calculated through a deterministic process, cannot, by definition, be random. Given knowledge of the algorithm used to create the numbers and its internal state, it is possible to predict all of the numbers returned by subsequent calls to the algorithm. For this reason the numbers produced by computer-based "random number generators" are often referred to as pseudorandom numbers. In the case of genuinely random numbers, knowledge of one number or an arbitrarily long sequence of numbers offers no clue as to what the next number to be generated will be. Humans are among the worst random number generators. Ask someone to pick a number "at random" between 1 and 20, and the number they are most likely to choose is 17. Psychologically random numbers, such as 17, are usually odd and don't end in 5, so that they frequently tend to be prime numbers.