Whether you're a kid in high school or someone trying to protect their stuff in a gym locker from getting stolen, having a master lock is critical. That said, they're not really the most secure devices out there.
Sure, you might think that your stuff's safe, but you wouldn't use a master lock to protect your diamonds and cash, would you? We certainly hope not.
To prove that point, here's a quick video on how to crack a master lock combo in three easy steps—which might make you buy a more secure lock that requires a key to open.
As someone who uses a master lock quite frequently, I've just given you the secret to stealing all my crap, so have at it!
While we wouldn't recommend you going around trying this, it's a pretty sly trick to have handy for when you have a master lock and don't know the combination—or you just want to show-off your master thievery skills.