Q.: Why is it considered necessary to nail down the lid of a coffin?

A.: This from a mortician: "When rigor mortis sets in, the fluid in the body becomes tight and constricts, which, on occasion, causes the body to move involuntarily. When a body is placed in a coffin, the lid is nailed shut to prevent an arm or leg from forcefully opening the casket. Although I imagine it was also done in the past to help deter grave robbers."

http://www.mindlesscrap.com/stumpme/12-02.htm