The Greek historian Herodotus reported that it took the labour of 100,000 men to build the Pyramid of Khufu at Giza. Since he was writing 2000 years after the event we cannot place much reliance on this figure. However, we can use physics to assess the accuracy of the account. As the pyramid was presumably built using human muscle power alone, it is possible to estimate the labour that went into its construction from the amount of potential energy that is stored in the pyramid (Nature, 383, 218, 1996) and the fact that a man can perform about 2.5 x 105 J of useful work in a day.