My first graduate microeconomics class didn't start with any grand theories or mathematical proofs. It started with a demonstration. The professor marched us down to a computer lab, divided us into groups of six, and had us participate in a simulated market.
Amazingly, the price of the virtual commodity we were trading quickly converged to a single value and stayed there. This was a powerful demonstration of the so-called law of one price, the economic principle that markets produce a single price for a given commodity or good.