Precipitation reaction and supersaturation?

[ASidd]

I read online that when you add a precipitating agent to a solution it initially becomes superstaurated and you have to take measures to decrease this supersaturation.

My question is why does it become supersaturated. Really having a hard time understanding this concept.

 

[Borek]

You need an activation energy to start nucleation (creating crystals). Sometimes this activation energy is so small, just thermal movements of molecules/ions in solution are enough to start precipitation, sometimes it is much higher and you need some stronger impulse - like a shock wave in the supersaturated solution of sodium acetate in hand warmers.