Okay, let me explain sokushinbutsu like you're five years old.
Sokushinbutsu is a really old way of making dead bodies look different than they did before. You know how sometimes people like to put makeup and nice clothes on dead bodies at funerals? Well, sokushinbutsu is like that, but way more tricky and serious.
People who wanted to be sokushinbutsu were usually Buddhist monks who wanted to show how dedicated they were to their religion. They would go through a lot of hard work and pain to make their body look different after they died. They believed that if they could make their body look like it was still alive, they would be more likely to achieve enlightenment (which is like becoming really, really wise).
So, to become sokushinbutsu, the monks would do lots of things to their bodies while they were still alive. They would only eat a little bit of food and drink almost no water for a really long time. Then they would start eating a special kind of food that was poisonous. This would make them sick and make their body feel really weird.
After that, the monks would lock themselves in a dark room and meditate for days and days, without any food or water. This was probably really hard and uncomfortable, but they believed it would make them closer to their religion and help them become sokushinbutsu.
Finally, when the monks died, their followers would do one last thing to make their body look like it was still alive. They would put the body in a really special kind of coffin and cover it with chemicals to dry it out. This would make the body look super skinny and wrinkly, but like it was still a person who was alive.
So, that's sokushinbutsu! It's a really old tradition that some Buddhist monks used to do to show how dedicated they were to their religion. It was really hard and uncomfortable, but they believed it would help them achieve enlightenment.