To make up for a shortage of stones and timber for building houses and temples, the Sumerians created molds for making bricks out of clay,

according to Kramer.

While they weren’t the first to use clay as a building material, “the innovation is the ability to produce bricks in large amounts, and put them together on a large scale,” Jones explains. Their buildings might not have been as durable as stone ones, but they were able to build more of them and create larger cities.