The Omnitrix

The Omnitrix

The main focus of the series is the Omnitrix, a watch-like alien device found by Ben in the first episode. Using it, Ben can transform into a number of different aliens, which he uses to battle the various villains in the series. As explained in the show, the Omnitrix works by binding alien DNA to Ben's, transforming him into the alien of his choice for a short period and then reverting him back to normal. As revealed in Ben 10: Race Against Time, the limited period in which it stays active (approximately 10 minutes) is a failsafe designed to keep the alien DNA from overwhelming the user.
To prevent the obvious problem of someone simply stealing the Omnitrix from Ben, the Omnitrix is depicted as quite difficult to remove. Furthermore, when anyone tries, it emits an omni-directional energy burst to discourage them. When people have actually removed the device, it seems to take them a very long time or complicated machinery to accomplish the task.
As a running gag, as well as an occasional plot point, the Omnitrix tends to work in ways contrary to Ben's wishes. It will transform him into the wrong alien, shut down at inopportune moments, or simply not activate at all. Various episodes show that at least part of this behavior is Ben's unfamiliarity with the device, and several episodes showcase people (or aliens) whose knowledge of the device allows them to operate it with much greater ease than Ben. When used to its full potential, the Omnitrix is shown to operate indefinitely and at will, rather than through Ben's ordinary manual selection. It will time out when Ben gets badly hurt, instead of ten minutes.
The Omnitrix's origins are explored in Ben 10: Secret of the Omnitrix. Its creator, Azmuth, explains that he built it to provide the various species of the universe a better understanding of each other — the proverbial 'walk-a-mile-in-another's-shoes'. To that end, the Omnitrix absorbs and integrates the DNA of aliens for use by the one wearing it. Because of the combat potential in some of the aliens, particularly those Ben commonly transforms into, it came to be seen as a weapon by Vilgax. Afraid that it would be used for the wrong purposes, Azmuth installed a self-destruct feature into the Omnitrix, which served as the central plot point in Secret of the Omnitrix Omnitrix
See also: List of Omnitrix aliens