It's an easy, but often overlooked, way to protect a home from unwelcome company.
But if not properly installed, a deadbolt-style lock is easier to manipulate than many realize.
"It's good to check it just to make sure its locking properly. It would only take you a few minutes to walk around your house," said Ron Sparks, a locksmith in South St. Louis.
A lock's effectiveness depends on two parts: the deadbolt and the mortise hole.
Unless fully extended, the deadbolt will not truly lock. It's very easy to push back into place.
The mortise hole is the gap in the door frame where the deadbolt can extend. If that hole isn't as deep, or deeper, than the deadbolt itself -- the lock is easy to manipulate.
Locksmiths recommend a mortise hole at least 1 inch deep.
"Even if you put a $500 lock on there, and [the mortise] isn't drilled deep enough -- it's still not locking all the way. And they're going to be able to pop it right open," Sparks said.
Recently, he realized his own mother-in-law, Pauline Brand, had a faulty lock. In her new home, the mortise hole wasn't drilled deep enough.
"It wasn't obvious," Brand said. "I had no idea that the lock was messed up that anybody could have just came in."
Sparks fixed Brand's lock immediately. He said it's easy for anyone handy with a drill -- or a professional -- to fix theirs, too.