This puzzle was a very very popular joke puzzle back in the 1920's in the US, particularly with salesmen and among athletic teams.
Once you have it down, you can quickly, in about five seconds, attach it to someone's dress shirt or polo. The faster you can attach it the less likely they'll see how you do it. Any buttonhole can be used from neck to waist or the ones at the wrists on dress shirts. After you attach it, just step back and see if they can solve it (they can't). Some people will never figure this one out, they'll be amazed when you can remove it easily.

This puzzle is 3 1/8” long, 1/4” wide, and comes in a set of 3 in assorted colors.