Okay kiddo, so let me tell you about the fall of Phnom Penh. Phnom Penh is the capital city of Cambodia, which is a country in Southeast Asia. Many years ago, there was a war called the Vietnam War, and Cambodia got involved in it too.
There was a group of people called the Khmer Rouge, and they didn't like the government of Cambodia. They wanted to take over and rule the country their own way. So, they started fighting against the government, and in 1975, they managed to take control of Phnom Penh.
Now, when the Khmer Rouge took control of the city, they did some really bad things. They didn't like people who were educated or rich, so they forced them to leave the city and go live in the countryside. They also didn't like people who were related to the old government, so they imprisoned and killed many of them.
Things didn't get any better for the people who stayed in the city either. The Khmer Rouge didn't believe in things like schools or hospitals, so they shut them down. They made people work really hard, and didn't give them much food to eat. Many people got sick and died because of this.
It wasn't until 1979, when a different group of people called the Vietnamese army came in and defeated the Khmer Rouge, that things started to get better in Cambodia. But it took a long time for the country to recover from all the harm that was done during the Khmer Rouge's rule.
So that's the story of the fall of Phnom Penh, kiddo. It was a really scary and sad time in Cambodia's history, but things are much better there now.