It was discovered around 6000 BC that wheat seeds could be crushed between simple millstones to make flour. The Romans were the first to grind seeds on cone mills. Most mills were driven by wind (windmills) or water. Later, they were driven by steam. In the 1930s, some flour began to be enriched with iron, niacin, thiamine and riboflavin. In the 1940s, mills started to enrich flour and folic acid was added to the list in the 1990s.