Acorns, also known as oaknuts, are the seeds of trees belonging to the oak family. All acorns contain tannins, which are substances that can be toxic to humans and other species, and which give acorns a characteristically bitter taste. However, levels of tannins in the acorn vary from species to species - tannin levels are fairly high in American red oaks and English oaks, while they are lower in white oaks. The tannins can be removed through a process called "leaching", which involves soaking the acorns in water; usually, as the tannins are leached out of the acorns, the water turns brown. When the water is brown, it should be changed, with the process repeated until it no longer changes colour. Leaching can be done in around an hour if hot water is used, but can take several days in cold water. Once the tannins are removed, the acorns can be used in many of the same ways as other nuts.