For use with water meters that require a special key to unlock, the key way will usually look like this:

Originally modeled after the exact tool featured in this video. Usage instructions also in the video.

You don't really turn it like a key, there should be a lever that pulls in the direction toward the street, or opposite of the key opening. Insert and press the lever by tilting the key.
These can be printed five at a time and make great freebies for neighbors if you leave a couple out during a winter storm.