Suppose you have been ship wrecked on an island. Desiring to return to civilization, you build a raft with a primitive sail to take advantage of wind power. Unfortunately, the wind blows randomly in two different directions. The first direction will drive your raft toward civilization. The second direction drives the raft further out to sea. What to do? When the wind blows in the first direction, put the sail up. When the wind blows in the other direction, reef the sail and wait for the wind to change. And so it is with trading.