The centre of the Milky Way is a very special place, harboring many exotic objects, such as the supermassive black hole Sagittarius A* and giant molecular clouds. Some of these clouds, despite being cold, are sources of high energy photons. It is believed that the clouds are not producing these photons themselves, but rather scatter the X-ray radiation coming from outside. Even though Sgr A* is currently very faint in X-rays, it is considered as the main culprit of this radiation, in the form of short but intense flares, which happened over the past few hundred years. The time delay caused by light propagation from Sgr A* to the clouds and then to us, allows one to study Sgr A*’s past activity. At the same time, flares serve as an extremely powerful probe of molecular gas properties. In particular, the full 3D structure of molecular clouds and their density distribution on small scales can be reconstructed.