This video explains how water can act as an acid or a base when it autoionizes or self ionizes. This is the process water undergoes when it ionizes into [H+] and [OH-] ions. It is the production of a hydrogen ion that shows how water can act like an acid, and the production of the hydroxide ion that shows how water can act like a base. Since it doesn't ionize much, it is considered a weak acid and a weak base.