This facebook post reminded me of a Joke

A young couple go into a cemetary at night, they start kissing and one thing leads to another and they decide to use a flat tomb stone to perform the "Act of Love".
The next day the girl is suffering from back and hip pain, she goes to the emergency room. 
The doctor has her disrobe, and procedes to check her back, legs, and hips. he has her doing range of motion tests then, in the middle of the examination he stops and askes the girl "How old are you?  She replies, "23".  The doctor tells her that according to her ass she died in 1826.