If you've ever accidentally locked yourself out of a room in your home that has a bathroom-style push-button or twist-privacy lock, you know it can be a pain to open them again. If you have a paper clip however, getting back in is no problem.
These tips from Instructables user DIYHacksAndHowTos will get almost any common indoor locked door open again in a snap, and all it takes is a paper clip. He walks through some common simple locks, including the dreaded privacy lock, and shows you how to open each one. Most of them aren't designed for security, just a little privacy or discouragement, so they're easy to open. With privacy locks, you just need to get the paper clip in deep enough to touch the pressure plate inside, and the door can be opened by twisting the knob.
Most household privacy locks have tiny keys that come with them, but if you don't know where those keys are, or your kid pressed the button-lock and then shut his bedroom door on the way out (which I've seen happen) this method will come in handy. For an added bonus, he even shows you how to open a pair of toy handcuffs, so keep that trick in the back of your mind for a rainy day.
Reminder: don't do anything dumb or illegal with the knowledge. I've locked myself out of rooms in my home before, so this is genuinely helpful, but don't go breaking into your roommate's bedroom or trying to get into someone else's home this way.
How to Pick Simple Locks/Latches with a Paper Clip [Instructables]