What stops planes from being stolen? Related question: do planes have keys?

Let's say I happen to spot a fine-looking plane at my local airport and decide the owner probably wouldn't mind if I borrowed her for a little while - what stops me from stealing this plane? On commercial jets, the cockpit door is locked, but as I understand smaller planes don't have cockpit doors. The linked question suggests that single-engine planes have ignition keys, but twin-engine planes don't. Are the outer doors (cabin doors) locked? What else restricts me from walking up, climbing in, and flying away? (Other than the fact that I don't have the least clue how to fly a plane.)

(This is about all types of planes.)
There are a several theft-deterrent mechanisms in place.


On small airplanes, most commonly, the door to the airplane itself is usually
locked with a key. (Notice the key cylinder in this picture below the silver door handle:
Prop locks:
Throttle locks:
A lot of airplanes do have an ignition key which you turn to engage the starter.  Of course, as jaytre mentioned, this can be circumvented by hand-propping (which is a very dangerous endeavor for unskilled people who haven't done it before)
You would have to know how to operate the basic systems of the airplane as well as how to fly.
AOPA's Airport Watch Program
Fences and other physical security measures around airports.