What causes hail? 

Hail begins as frozen raindrops of snow in a thunderstorm cloud.  These pieces of ice are carried by updrafts to a part of the storm where droplets of water exist in a supercooled state.  These supercooled (actually in a liquid form at temperatures below freezing) droplets freeze into ice when they strike the surface of the pieces of ice. As the ice accumulates, the pieces of ice grow into a hailstone. It continues to grow larger and larger the longer it stays among the supercooled droplets.  As this hailstone gets heavier, it falls, but other currents carry it back up. Eventually, the hailstone becomes too heavy to be supported and falls to the ground and does all sorts of lovely things to cars and houses.