Into how many pieces does a balloon burst?

A moderately inflated rubber balloon pricked with a needle bursts into two large fragments. However, if you inflate it until it bursts spontaneously, you get dozens of shreds.

Now, Sébastien Moulinet and Mokhtar Adda-Bedia at the Laboratoire de Physique Statistique (CNRS/UPMC/ENS/Université Paris Diderot) have explained this phenomenon: when a crack propagating across the surface of a balloon reaches a critical speed, it becomes unstable and splits into two new cracks.

It is this mechanism of proliferating cracks that causes the balloon to burst into shreds. The work, published in Physical Review Letters, sheds light on the fragmentation processes in materials subjected to impacts or explosions.