Here comes the science bit
Dissolving a salt, such as Epsom salts, in water forms a solution. Heating the water creates space between water molecules so that the water can hold more salt. When the hot water won’t dissolve any more salt, the solution is called “saturated.”
As the saturated solution cools, the salt is pushed back out of the water as a solid. As the water evaporates, the salt crystals are left behind. When the water cools, the crystals will fall out of the solution and collect at the bottom of the bowl.