A long, long time ago, there were some people called Xiongnu who lived in a place called Central Asia. They were really good at riding horses and shooting arrows, and they were very strong and brave. They would ride around and conquer other people and take their stuff.
One day, some of the Xiongnu decided to move to a place called Mongolia. This is where they met other people called Mongols, who were also good at riding horses and shooting arrows. The Xiongnu and the Mongols liked each other and became friends.
Over time, the Xiongnu and Mongols started to share their ways of living and fighting. They learned from each other and became even stronger together. Eventually, the Xiongnu and Mongols became so powerful that they were able to conquer many other kingdoms and territories.
So, the Xiongnu brought their skills and knowledge to Mongolia and helped shape the people who would become the famous Mongol warriors we know today.