  A drop of water that freezes from the outside-in presents an intriguing
problem: the expansion of water upon freezing is incompatible with the
self-confinement by a rigid ice shell. Using high-speed imaging we show that
this conundrum is resolved through an intermittent fracturing of the brittle
ice shell and cavitation in the enclosed liquid, culminating in an explosion of
the partially frozen droplet. We propose a basic model to elucidate the
interplay between a steady build-up of stresses and their fast release. The
model reveals that for millimetric droplets the fragment velocities upon
explosion are independent of the droplet size and only depend on material
properties (such as the tensile stress of the ice and the bulk modulus of
water). For small (sub-millimetric) droplets, on the other hand, surface
tension starts to play a role. In this regime we predict that water droplets
with radii below 50 micrometer are unlikely to explode at all. We expect our
findings to be relevant in the modeling of freezing cloud and rain droplets.
