Hail forms inside powerful thunderstorms with strong updrafts - rising columns of air. A small ice particle gets pushed up, encounters supercooled water droplets (water that’s below freezing but hasn’t frozen yet), and gets coated with another layer of ice. Then it falls. Then the updraft catches it and pushes it up again, where it picks up another layer. Then it falls again. Then up. Then down. And so on.
Each cycle adds another layer to the hailstone. Slice a big hailstone in half and you can see the layers like an onion. By the time the hailstone gets too heavy for the updraft to lift, it falls to the ground - sometimes the size of a pea, sometimes the size of a baseball.
The biggest hailstone ever measured was found in South Dakota in 2010. It was 8 inches across and weighed almost 2 pounds. Hail that big falls at over 100 mph and can punch right through roofs and car windshields. Large hailstorms cause billions of dollars in damage every year, mostly to crops, cars, and roofs. Birds have been killed by hail. Even people, occasionally - a hailstorm in India in 1888 killed over 240 people with hailstones reportedly the size of cricket balls.
