When you are walking through the Old Medina in Fès, make sure you ask your guide to explain how the medieval water clock works. He loves a challenge! Constructed in 1357, the Dar al-Magana served to regulate prayer times. But the complex – and quite ingenious – combination of water reservoirs, metal balls, gongs, keyhole windows, pulleys and hanging weights that made the clock tell the time has remained something of an enigma for centuries. Scientists (and tour guides) have only recently discovered how the fascinating mechanism actually works.