This is the classic Haunted Key magic trick.  Im not giving away the secret, but if you know, you know.  Its easy enough to figure out if you use google.  The key mysteriousely rotates in your hand, and then you can hand it out for examination.  Its hollow on the inside, so you can put in 2 40mm rods to give it some weight.  (Optional). just glue the pieces together, and you are good to go.