Long ago, in the wide grasslands of West Africa, two kingdoms grew so rich and so famous that travelers told stories about them for hundreds of years. Their names were Ghana and Mali. So what made them legendary? Two little things, mostly: gold and salt.

Here's the funny part. In one region of West Africa, gold was almost everywhere underground. But there was barely any salt. In another region, far to the north in the desert, salt came in giant slabs — but no gold. People in both places wanted what the other had.

Why was salt such a big deal? Because your body needs it, and in a hot climate you sweat it out fast. Without salt, food spoils and people get sick. So in the desert, salt was so precious that people sometimes traded it for an equal weight in gold. Yes — salt for gold, slab for slab.

The kingdom of Ghana sat right in the middle of this trade — like a toll booth on the world's busiest road. Camels carried salt down from the north and gold up from the south, and everything passed through Ghana. The kings simply taxed the traffic. That made them wonderfully rich.

After Ghana came an even greater kingdom: Mali. Mali grabbed the gold mines AND the trade routes, so it became one of the richest places on Earth. Its most famous king was Mansa Musa, and "Mansa" simply means "king." Mansa Musa may have been the wealthiest person who ever lived.

In the 1300s, Mansa Musa took a long journey across the desert to Mecca. He brought a procession so enormous — camels loaded with gold, thousands of people — that towns gasped as it passed. He handed out so much gold along the way that, in some cities, gold actually became less valuable for years afterward.

But Mali wasn't only about money. At the edge of the desert sat a city called Timbuktu, and it became one of the great learning centers of the world. Scholars there studied math, medicine, astronomy, and law, and they filled libraries with thousands of handwritten books.

One building in Mali tells the whole story at once: the Great Mosque of Djenné. It's the largest mud-brick building on the planet, built entirely from earth and wood. Every year the whole town gathers to re-plaster it by hand, smoothing on fresh mud so it never washes away.

So when people remember Ghana and Mali, they remember more than treasure. They remember clever kings who turned a road into a kingdom, scholars who filled the desert with books, and builders who made marvels out of mud. Two of the great kingdoms of the world — born from gold, salt, and very good ideas.

And it all began with the two simplest things in the world. One you dig from the ground and one you sprinkle on your dinner. Funny to think that an entire empire once stood on a pinch of salt.