Keys are typically little pieces of brass. While this alloy of zinc and copper has many virtues, one of the drawbacks is that it will fatigue and tear with over- or misuse.
When a key breaks, it almost always breaks inside of a lock.
In this situation, you can call a locksmith and have them extract the broken key. This, however, is usually like calling a plumber to pump a plunger in your toilet for a few seconds. Key extractors are inexpensive and simple to operate.
Broken keys will still work (probably)
If a key breaks in a lock, you can usually push the broken piece into the lock with whatever remains in your hand and turn the lock. Yes, broken keys will frequently work.
The problem then becomes getting the broken bit out. Pushing that broken piece deeper into the lock to open it only makes it harder to reach.
Even if the lock still works with a segment of broken key in it, you’re going to need to remove it. That broken piece might not continue to work and if it does work, it is compromising your security.
Key extractors come in all sorts of shapes, but there are two common forms: harpoons and hooks. Both are tiny and sharp.
Both forms of extractors have their virtues and can get the job done. Most locks found in residences and escape rooms within the United States have wide-open keyways, so the shape of your extractor doesn’t matter so much. It only matters that it’s tiny and sharp.
To operate a key extractor, stick it into the keyway and push all of the pins up as high as they’ll go with the extractor. Try to get one of the sharp prongs to hook a key bit. Then pull it out.
For a visual lesson, the incredibly knowledgeable and talented Bosnianbill explained and demonstrated this in a video:
Damn near every other thing that you’d call a locksmith for requires some level of skill (and frequently a lot of skill). Extracting keys is usually a simple process… unless it isn’t and things are all messed up… Then call a professional.
Good extractor options
Most people probably do not need a key extractor, but escape rooms should absolutely have them handy. Key extraction is better than bolt cutting a padlock or disassembling a door lock.
Key extractors are like screwdrivers. You can buy them in sets and then grab the one that best fits the lock at hand.
I keep a set of Peterson Saw-Tooth Extractors in my pick kit. The set cost $20, was manufactured in the United States, and works well. Incidentally these key extractors also work well for removing micro sim cards from micro SD ports.
If you’re dealing with narrow keyways particularly common to European locks, or you want additional options, a German-made Multipick Extractor-Set 4 pieces ELITE set would be a great option. This will run you €28.
Whatever you do, do not buy a key extractor or any other locksmithing tools on Amazon. For reasons that are not particularly clear to me, basically all of the lockpicks and related tools sold on Amazon are garbage.