Mark, there is this game, its called "riding the bull"

What you do is this..

have sex with your girlfriend in the doggy style position...
grab onto her hips and start to really work it
then scream out her sisters name (or her best friend's name) and see how long you can stay in her... if you can stay in her until you cUm then you complete the ride..
if you get bucked out.. well, you loose... either way, she'll probably scream..


: )