A super saturated sodium acetate solution is poured onto a sodium acetate seed crystal. The seed crystal provides a crystalline structure template for the sodium acetate ions to form on. This is an exothermic process.
This demo shows how a precipitation reaction works. The precipitate formed is a common ingredient in chalk.
CaCl2 (aq) + Na2CO3 (aq) → NaCl (aq) + CaCO3 (s)