DEMAND 需 - my story is based on Elphalpho's one.
In this demonstration of supply and DEMAND, an economics professor eschews the typical widget, and uses COMBS as his sample product. He then goes on to show that if it RAINS, the demand for COMBS will rise. However, if it RAINS COMBS, the DEMAND for COMBS will fall. It's all about supply and DEMAND.