Appliance of science: What happens when you split an atom?

My nine-year-old is obsessed with atoms and that has extended to questions about what happens when you split them, how does it cause an explosion and can atoms be split in space? What are atoms?

Before talking about what happens when you break an atom apart, it would help to understand what an atom is and what they are made of. Atoms are extremely small objects that make up everything around us, they are the basic building blocks for all matter.