Every three Earth days (or 72 hours), Yeerks need to leave their hosts' body to enter the Yeerk pool and absorb Kandrona. Kandrona is naturally produced by the sun on the Yeerk homeworld, but Yeerks have developed artificial Kandrona Ray sources (which can be large and serve a large area, or small and portable) which allow them to travel away from their planet. Without Kandrona, a Yeerk will die.