There were many countries in Europe who were very upset with each other after World War II. They wanted to work together so that they could avoid fighting each other again in the future. So they decided to form what they called "European Communities."
At first, there were only six countries that were part of the European Communities: France, Germany, Italy, Belgium, the Netherlands, and Luxembourg. They wanted to make sure that they could trade with each other without having to worry about things like taxes or customs. They also wanted to help each other out if there was ever an emergency, like a natural disaster or a war.
It was a bit hard for some of the countries to work together because they had been enemies in the war, but they tried to put that behind them and focus on what they could do together. They built things like roads and bridges to connect their countries, and they worked on making their economies stronger by creating jobs and making it easier for businesses to trade with each other.
Over time, more countries joined the European Communities, and they started working on even bigger projects. They made rules for how things should be done, like how much money countries should spend on things like healthcare and education. They also made big decisions together, like whether or not to allow other countries to join the group.
By 1957, the European Communities had grown so much that they decided to become more official. They signed something called the Treaty of Rome, which made it clear that they were all part of one big group working together. People were happy because by working together, they were making their countries stronger and more peaceful. And that's how the European Communities got started!