Magic balloon

The rubber in the balloon consists of many long molecules (polymers) that are linked together (cross-linking). These links hold the polymer molecules together and allow them to stretch up to a point. When the force or tension applied on the cross-links is too great, they will break, and the polymer will pull apart.

The trick is to put a small piece of sellotape tape. The tape sticks to the rubber in the balloon and will not allow the rubber to stretch to the breaking point when the needle pierces the balloon (the tape reinforces the cross links, and the balloon stays together).