Once drops to , particles start to fall into the ground state and continue to do so as the temperature continues to fall. Contrary to the naive expectation at the outset of this discussion, we never actually reach a state where all of the particles are in the ground state for temperatures above absolute zero. Instead, the system crosses over having no particles in the ground state to having some . It turns out that this crossover has a distinctive signature in the system's thermodynamic properties, and therefore constitutes a phase transition. This is an important point.
Phase transitions involve changes in thermodynamic properties.