A5135 Small Lock-style Solenoid 6VDC @ 600mAh
Solenoids are basically electromagnets: they are made of a big coil of copper wire with an armature (a slug of metal) in the middle. When the coil is energized, the slug is pulled into the center of the coil. This makes the solenoid able to pull from one end.
This solenoid in particular is nice and strong and has a slug with a slanted cut and a good mounting bracket. It's basically an electronic lock, designed for a basic cabinet or safe or door. Normally the lock is active so you can't open the door because the solenoid slug is in the way. It does not use any power in this state. When 6VDC is applied, the slug pulls in so it doesn't stick out anymore and the door can be opened.
The solenoids come with the slanted slug as shown above, and unlike the larger version, you cannot turn the slug end around!
You'll need to power this here solenoid with about 6VDC (you don't have to be exact, 5 to 6V will work fine) and ka-pow, it's now a magnet. Turn off the power and it reverts back to a normal chunk of metal. Since it's a coil, you'll need to use a motor or solenoid driver with kick-back protection. These draw quite a bit of current, 500mA at 5V and 600mA at 6V so you'll want a good strong driver.