An Allen wrench is one of the simplest wrenches to use. The Allen wrench itself is a small L-shaped wrench with six sides. If you look at a cross-section of the Allen wrench, it looks like a hexagon. Since the Allen wrench has such a specific shape, it can only be used with items especially designed for it. If you've ever bought furniture you've had to put together yourself, the manufacturer likely included an Allen wrench for you to use when you have to assemble it.
In addition to assembling furniture, the Allen wrench is also commonly used for bike repairs. The unusual shape, plus the fact that the Allen wrench is small and light and capable of getting into hard-to-reach places on a bike, makes it particularly useful for cyclists to carry with them as they ride.
When you need to use the Allen wrench, stick the short end of the L shape into the bolt head and press down on the longer side of the L to turn the bolt. Of course, the Allen wrench will only fit in a hex bolt (a bolt with six sides), so don't try using the Allen wrench to screw in any other type of fastener.
The other most common way an Allen wrench is used is to pick a lock. Allen wrenches come in a range of sizes, so as long as you have one small enough to fit into a lock for which you've lost the key, you can use it to try to open the lock. Simply stick the end of the Allen wrench into the key tunnel and…sorry, we're not going to teach you how to pick a lock!