It is fascinating to watch an anthill. Thousands of ants are extremely busy, and show an extraordinary mobility drift. Their movements may sometimes look a little bit chaotic, but a closer inspection brings to light that they all follow certain rules. Their apparent chaos hides rational behavioural patterns instigated by their sense of collective survival. It is equally fascinating to watch a modern large airport. There, thousands of people are moving around, in all directions, and the spectator gets the superficial impression of a strange chaotic system. But, here again, we know that this seemingly chaotic mobility pattern is governed by the strict behavioural rules of air travellers – mostly tourists – who are rationally seeking to reach their final destination. The difference between ants and airline passengers is perhaps that ants do not know how to relax, whereas tourists seek to relax through their trips. But in all cases, we observe mass movements that are induced by rational choice mechanisms.