In the past few decades, black holes went from being theoretical concepts in the mind of a few humans, to being fundamental subjects of modern astrophysical research. Invisible by definition, black holes exert a profound influence on the matter which approaches their event horizon, heating it up until temperatures of millions of degrees. Such extremely hot matter shines in X-rays: it is thus through X-ray photons collected by large space telescopes orbiting Earth, that cosmic black holes can be investigated nowadays. After 20 years of scientific operations, the largest X-ray telescope ever flown — XMM-Newton — had collected thousands of signals from cosmic black holes, but none of them was similar to the one registered on December 24, 2018. On that day XMM-Newton was observing the galaxy GSN 069, when a completely unexpected cosmic signal was registered. Every nine hours, the nucleus of the galaxy brightened up in X-rays by a factor of 100, for about one hour: X-ray Quasi-Periodic Eruptions had been discovered. In this talk I will introduce the basics of X-ray observations of cosmic black holes, in order to be able to tell you the story of the discovery of X-ray Quasi-Periodic Eruptions, and what they tell us about the activity of cosmic black holes.