It seems like an innocent Christmas card. Open it up and it plays a Christmas tune. The thing is it doesn't stop! It plays non-stop until the battery dies! You could just tear the card apart to stop it playing, but then you would discover that inside the card there is... glitter! Probably best to pick your victim, sorry recipient, carefully!
When you get the card, you can safely open it and write a message inside. Once you are ready, you pull out the tab, and now the card is 'primed'. Pop it in the envelope, and send it, or give it, to the recipient. The card is supplied with a regular envelope, but if you are sending it through the post, it is probably best to put it in something that will protect it, either a card backed envelope, or a bubble envelope.