The little office/sacristy where I sit when I'm volunteering for the Missionaries of Charity has its own bathroom. Tonight one of the residents (tall and very pregnant) arrived with a friend and asked if her friend could use it. No problem-- the resident waited for her friend, who closed the door behind her when she came out.
About ten minutes later the resident came back. "Could I use the bathroom myself?" "Sure!" She reached out and tried to turn the doorknob, but couldn't. Her friend had locked the door behind her.
We both stared at the door for a second. I certainly didn't have the key, and the sisters were off praying and not to be disturbed. "Oh no..." moaned the resident, "she probably didn't even know she did it..." She paused. Then she reached out for some 3"x5" cards on the desk. I thought she was going to leave a note for the sisters, but instead of grabbing a pen, she ran the cards down between the door and its frame. Click. It was open.
"I'm not a thief!" she laughingly assured me as she disappeared into the bathroom, and I was left feeling embarrassed that I, a Caltech alum, had not known that simple lock-picking trick. I'll remember it, though. :)