Sumer grew crops on the Tigris and Euphrates rivers. A surplus in food led to a Division of labour. This means that some people were able to stop growing crops and do other jobs, since enough crops were already grown. This brought a split in society. Today, such a split is called "social pyramid". In a social pyramid, people are grouped into social classes based on their wealth and power. In Sumer, the king, priests, and government officials were at the top of the social pyramid. Below them were the artisans, merchants, farmers, and fishers. At the bottom of the pyramid were slaves. Slaves were often prisoners of war, criminals, or people working to pay off debt.