A man and a woman who have never met before find themselves in the same sleeping carriage of a train. After the initial embarrassment, they both manage to get to sleep; the woman on the top bunk, the man on the lower. 
In the middle of the night the woman leans over and says, "I'm sorry to bother you, but I'm awfully cold and I was wondering if you could possibly pass me another blanket." 
The man leans out and with a glint in his eye said "I've got a better idea ... let's pretend we're married." 
"Why not," giggles the woman. 
"Good," he replies. "Get your own blanket."