Why does a Cheshire cat grin?

A possible origin of the phrase is one favoured by the people of Cheshire, a county in England which boasts numerous dairy farms, hence the cats grin because of the abundance of milk and cream. To grin like a Cheshire cat means to smile broadly. Some definitions of the term stipulate that the smile must be so broad as to expose the gums. The idiom grin like a Cheshire cat may have the connotation that the person who is grinning is in possession of knowledge that the beholder is not aware of.