Criticality accident

Okay, so you know when you play with blocks and sometimes you pile them up so high that they all fall down? Sometimes, when people are working with really tiny building blocks called atoms, they can accidentally pile them up too much and they can all fall down too. When this happens, it can create a really big explosion or a lot of heat and radiation that can hurt people nearby. This is called a criticality accident. It's like when you play with blocks, but you have to be really careful with atoms so this doesn't happen.