Hail is created when small water droplets are caught in the updraft of a thunderstorm.
These water droplets are lifted higher and higher into the sky until they freeze into ice.
Once they become heavy, they will start to fall.
If the smaller hailstones get caught in the updraft again, they will collect more water on the way up.
As the stone gets higher and higher the water will freeze on the outside of the ice pellet, causing the hailstone get bigger.
Once the hail stone become heavy enough it will fall towards the surface.
Depending on how many times this processes happens, before the hailstone finally falls to the ground, will determine how big the hailstone will ultimately be.