Acorns traditionally have been used as food by the native peoples of North America. Once taken out of the shell and crushed to the smallest particle size possible they must be processed to remove tannin. This can be done through immersion in multiple changes of cold water or running through cold water if held in a tight sack or basket or through boiling in multiple changes of water.

Thanks to http://www.practicalprimitive.com for the excellent workshop.