The threshold object I have chosen to discuss is a streering wheel (which is ordinarily a part of some sort of console). I admit that this is a fairly obvious choice, as it has been used in countless racing and driving games. I have also played a number of adventure and role-playing games in which some type of driving or navigation was involved, and most of these games made use of steering wheels for this purpose. Almost everyone is familiar with the things, and they're fairly intuitive; it doesn't take a computer science degree to drive a car. (Though sometimes, when driving around Boston, I wish it does.)
The steering wheel works to immerse an interactor in an environment by offering the interactor direct control over the situation. A driving game on one's computer, one that uses only cursor keys, misses some of the sensation that people get when actually driving a car. We know it's a computer, because no real automobile is controlled with a few buttons alone. On the other hand, an arcade game with a steering wheel gives the interactor the impression that he or she is actually driving. After all, there's the steering wheel, therefore you must be in a car!