Ultra-short and extremely strong X-ray flashes, as produced by free-electron lasers, are opening the door to a hitherto unknown world. Scientists are using these flashes to take 'snapshots' of the geometry of tiniest structures, for example the arrangement of atoms in molecules. To improve not only spatial but also temporal resolution further requires knowledge about the precise duration and intensity of the X-ray flashes. An international team of scientists has now tackled this challenge.