Temperature is average molecular kinetic energy. In ice hydrogen bonds lock molecules into a hexagonal lattice — they vibrate but can't swap places (which is also why ice is less dense than water). In liquid water the bonds keep breaking and re-forming: molecules stay close but roam. In steam the bonds are gone; speed wins and the gas fills the box.

Watch the readouts while you slide through 0 °C and 100 °C: around those points added energy goes into breaking bonds (latent heat) rather than raising speed — the plateau where ice melts at 334 kJ/kg and water boils at 2,257 kJ/kg. One more anomaly to try: freeze the box and notice the lattice hold its shape; the same molecules, different organisation, entirely different material.