A yo-yo is pulled by its string on the floor without slipping. Depending on the angle of the string with respect to the horizontal, the yo-yo will travel towards the puller or away from the puller.

Gas prices are on a yo-yo string. Two weeks ago, they were down. Last week they were up. Now they are down again. Prices at the pump on Monday, Feb. 17, have plummeted an average of 14 cents in ...