China's central bank is sitting on a giant pool of U.S. dollars. It's the world's biggest holder of foreign reserves, worth over $3 trillion at last count.

All that money has piled up because every year, China exports more than it imports; it runs a trade surplus.

There are lots of reasons for China's trade surplus. In the past few decades, China has built an amazing manufacturing ecosystem. It's become the factory to the world.

But China's central bank has also given the country's exporters an extra boost.

It has kept China's currency artificially weak relative to the dollar, which makes China's exports cheaper in the rest of the world. It also makes imports more expensive in China.

So keeping the currency weak increases China's trade surplus on both sides of the ledger — it increases exports and decreases exports. And, year after year, China's pile of dollars keeps getting bigger.