One of our family members has a serious water thief problem -- her neighbor stole over $800 worth of water a few months ago while she was out of town, although it was impossible to prove to try to prosecute the lowlife. I ordered these to put on her outside faucets, plus a couple for my own home. This device is both simple and effective -- it comes with a screw-on plastic cap, which does have a washer in it to prevent leaks if the water is turned on with the lock installed. I tested the effectiveness of the cap and with the water turned on full force not a drip came out. The second component of this gizmo is a metal security mechanism that opens up and closed across the threads of the hose bib (after you install the screw-on cap). It does NOT come with a lock -- you'll need to make a run to the store for that (or purchase one here on Amazon separately).This device is a good deterrent, but like all of these things not a total prevention device. If someone is really intent on stealing your water they're probably going to break the lock you put on it, or possibly use a power tool to try to saw off the locking mechanism. The best way to prevent water theft is to turn off the valve servicing the outside hose bib -- although many newer homes don't have these secondary valves anymore, which means you'd have to turn off ALL the water going to your house to stop the theft.