What Am I?

The third graders are working on making inferences. What do YOU think this riddle poem is about? Leave your guess in the comments!

Fitting In

I fit inside your door just right
wherever you reside.
But put me in your neighbor's door
and you'll still be outside.

There's just one hole that I fit in
made specially for me.
There's just one door, one safe,
one skate and one locked diary,

which makes me feel real special.
like I am one of a kind.
Of course, sometimes folks copy me
in case I'm hard to find.

I share a ring with others
of my type; I'm rarely single.
But that's okay 'cause when we
touch each other, then we jingle.

WHAT AM I?
(C) 1998, Arden Davidson