My Grandmother carried a key on her keychain that was a key to her heart... she shared it with Grandpa. So, one day, I asked him if I could have one too. He had boxes of old keys, through which he searched until he found "my key." I have treasured it ever since; only sharing it with my love, Joel.
I love this tradition of the locks at the Hohenzoller Bridge (Germany) - the bride and groom add a lock to the Bridge then throw the keys into the water below as a symbol of their union.