A friend some years ago had a little balsa wood box with a red button that said "Do Not Push."
When someone (inevitably) pushed the button, a little stamp pad popped up and made a red circle on the victim's fingernail, and then it all snapped back shut. (The entire internal mechanism consisted of two pieces of bent paper clip and a rubber band, with a short bit of 1/4 inch dowel for the "stamper.") We told the curious ones they'd just received a "red badge of courage."
A less friendly acquaintance came in later with a similar little box from a local "magic shop," with a similar little red button similarly marked, but when you pressed the button on his the button went down, but the needle in the middle of the button didn't. He was properly informed that it was not a particularly funny joke, and both he and his box disappeared shortly after (for reasons in addition to the box).
But of course both of those failed the "useless" test since they both actually did something.