Ever needed to lock the deadbolt behind you but didn't have a key to the door? Admittedly, this isn't a problem you'll get into every day, but Instructables user Hextor has a clever solution should you ever find yourself in this situation.
How did he come to it?
Long story short, I was brand new in my office, they hadn't given me an office key yet, and I was the last one in the office. [I either had to]wait 4 hours for a coworker to return or find a way to lock the door.
Jump a few years into the future, and I just recently found myself in the same circumstance, last one in the office with my wife's car (and keys) so I had no office key.
With a little help from some packing tape, the second time around Hextor was on his way in 5 minutes. Just affix the tape to your deadbolt lever on the inside of the door, fold it around the door, close the door, pull the tape, and voilà—you've locked the deadbolt. Simple enough, but clever nonetheless.
Locking a deadbolt from the outside without a Key [Instructables]