April is the hottest month in Thailand. Temperatures climb above 35 degrees, and the whole country is feeling the heat. So throwing water at each other is partly just a brilliant way to cool off. Children take it the most seriously - whole streets fill with kids armed with water pistols, the bigger the better.
The water has a meaning, too. In Thai tradition, gently pouring water over someone's hands at new year is a way of wishing them good luck for the year ahead. Older relatives stand still while younger family members pour a little water over their fingers. Then everyone runs outside and the polite version turns into the big version.
Songkran lasts three days, but in some cities (especially Chiang Mai) it goes on for nearly a week. Pickup trucks drive slowly along the streets with huge tanks of water in the back, and the whole family rides on top with buckets, soaking everyone they pass. Strangers smile and laugh as they get drenched. Then they soak the truck back.
It is the time of year when Thai people travel home to be with family. Schools and most offices close. Grandparents who haven't seen their grandchildren for months get a visit. Even people who don't go out for the water fight will spend the days cooking, eating together, and tidying their houses to start the year fresh.