The first department is the very first group of people that started working together to help keep a community safe. Imagine you and your friends live in a little house in a village. One day, a big storm hits and your house catches on fire! Your parents call for help and before you know it, some adults come running with big buckets of water to put out the fire.
These adults are part of the first department in your village. They are called firefighters and their job is to help put out fires and keep everyone safe. They work together as a team and use special tools like hoses and trucks to make sure the fire doesn't spread.
Over time, other people realized that they needed help too. They started creating their own departments to help with different things like treating sick people, catching bad guys, and making sure the streets were safe to walk on.
But it all started with the very first department - the firefighters - who were willing to come together and help out in a time of need.