It's amazing how many "problems" inventors around the world have attempted to solve with their creations. This one is truly ridiculous. Useful, but ridiculous. 

Several years ago a radio station I worked at was giving away these round plastic things with a slot along one side. Inside the slot is a small protruding edge. I have handed this item to many people throughout the years and asked them what they think it is. No one ever knows. This morning yo-yo was guessed, along with "dental floss holder." It's not a yo-yo, and it doesn't hold anything. In fact, this weird round thing is entirely complete as it is. There are no additional parts or accessories.

Do you know what it's for?