Nuclear fission is a process in which a heavy nucleus split into two. Most of the actinides nuclei (plutonium, uranium, curium, etc) fission asymmetrically with one big fragment and one small. Empirically, the heavy fragment presents on average a xenon element (with charge number Z=54) independently from the initial fissioning nucleus. To understand the mechanism that determines the number of protons and neutrons in each of the two fragments has been a longstanding puzzle.