Okay kiddo, I'll explain the San Ysidro McDonald's Massacre to you in a simple and easy way.
A long time ago, in 1984, there was a very sad event that happened in a place called San Ysidro in California. A man named James Huberty was feeling upset and angry about his life, and he decided to go to a McDonald's restaurant with some guns.
When he got to the restaurant, he started shooting people who were there to ate their food. Many people were scared and didn't know what to do, so they tried to hide or run away. James Huberty kept shooting for a very long time, until the police came and stopped him.
The massacre was very bad and made a lot of people feel very sad. Many people got hurt and some even died. It was a really scary and tragic thing to happen. That's why we always need to be kind to each other and try to solve our problems in a peaceful way instead of using violence.