Crystals are structures formed from a regular repeated pattern of connected atoms or molecules.
You can find more information about crystals here.
Growing crystals is a very popular activity in science lessons, ranging in complexity from simple crystal growth commonly encountered in primary schools , to more complex crystal formation such as growing crystals in gels.
You can find more information on crystals here.
The document includes: simple crystal growing, crystal gardens, crystals in gels and recrystallising for purification.
(Image by Tess Watson from Edinburgh via Wikimedia commons)