Question: why does crystalisation happen within the urine?

Susana Teixeira answered on 17 Jun 2013:

It is easy to understand if you first think about crystals… One of the things that favour crystallisation is high concentrations: when many molecules of the same compound are near each other, they are more likel to align in a 3D array where they are all facing the same direction, oriented in the same way way.

Urine can have a very high concentration of many materials, so if the right pH and temperature are present, crystals form. The good news are those crystals are usually expelled with the urine before they get a chance to grow too large!