The condensation Einstein found derives from the fact that, in the limit of an infinite three-dimensional volume, the total number of states at vanishing energy becomes exceedingly small. Thus there is not room for all the particles when the temperature is decreased, and the system can only accumulate all the superfluous particles in its very ground state; they condense into the lowest energy state. In the thermodynamic limit, when both the particle number and the volume grow to infinity, the system enters into a different state, thus undergoing a phase transition. For a long time, no physical system was known which would display this interesting phenomenon.