Ping wrote:


This teaches most people a important lesson: US dollar do change, we shouldn't buy US dollars in just for keep the price. The only stayble money is gold and silver.



Until recently, China held it's currency in step with the dollar and did so artificially.  This allowed chinese imports to be cheaper than US products.  But it also gave chinese investors some pretty strange expectations of the value of the US$.

All money fluctuates in value over time.  If you want to make a profit, buy when the dollar is low vs. the yuan and sell when the dollar is high.