The Treaty of Lyon is like an agreement or a promise that two countries, France and Spain, made to each other a long, long time ago, in the year 1601. You know how sometimes you and your friend may argue over whose toy it is and you both feel upset? Well, France and Spain had something similar happen to them but instead of toys, it was about the land they both claimed to own. This made them very unhappy, so they decided to have a meeting and talk it out.
During the meeting, they talked about the land they each believed belonged to them and how they could share it in a fair way so that they would both be happy. It was like they were playing a game and they wanted to come up with a way where both of them could win.
Finally, they agreed on something called the Treaty of Lyon. This treaty said that the land they were fighting over would be divided in half and each country would get one part. They also agreed that they would be friends and would not fight again. It was like they both got what they wanted and no one was angry anymore.
The Treaty of Lyon was a very important event because it showed that countries could talk and work together to solve their problems instead of fighting and being angry. And this is why it is remembered even after so many years.