The Number Gym is a website that has won compliments from adults and children alike. Melvyn Rust must have enjoyed devising this collection of interactive mathematical games for children of primary age, and the sense of fun comes across in the "Whee!" with which the computer celebrates a correct answer, whether you're calculating against the clock or working out what fraction or percentage of a chocolate bar remains to be eaten. The Number Gym is free to use on Fridays and at weekends. Software can also be purchased.