It's the balloon under the shirt, and the balloon is red, but when you pop it with the pin, it's not red at all. The balloon is loose, and she's trying to figure out how to get it back on. The trick is to cut the balloon in half, and then inflate it with a new one. The audience will see the two halves of the balloon, and they'll be able to see that the balloon has been cut in half. Then, the audience will have to clap to make sure that the new balloon is inflated correctly. This is a great way to cheer up the audience, because they're not going to boo you when you're done with the trick.