We just learned that rain happens when enough water vapor collects in a cloud and it falls to the ground as water.
If there is a thunderstorm cloud and the air in the sky is cold enough, this can cause hail.
The water in the clouds freezes and turns to very small bits of ice.
In a cumulonimbus cloud with a lot of strong wind blowing up, the small bits of ice will keep getting blown back up into the sky and then falling down, getting more and more ice on them until they are too big and heavy to get blown back up and they fall down to the ground.
Hail can be as small as tiny pebble or larger than a golf ball!
(from: wikipedia - hail)
Kid Facts - Blast from the past: Minerals
