Hail is a shower of frozen raindrops that are created when small water droplets are caught in the updraft of a thunderstorm.
The fast winds inside the large cumulonimbus clouds blow the raindrops so high that they freeze into small balls called hailstones.
As the hailstones swirl around inside the clouds they pick more water.
This water freezes on the outside of the stones creating a layer of ice around them, making them grow larger and larger.
Once the hailstones become heavy enough, they will fall from the cloud to the ground.