For years, a California warehouse had stored freshly picked green lemons before they were shipped to market. The warehouse managers knew that the lemons would always be ripe and yellow and ready to ship to market about five days after they arrived. Or so they thought. One year, for safety reasons, they decided to replace the warehouse's kerosene heaters with modern electric ones. Then, much to their surprise, when they began to pack their first shipment of five-day-old lemons, they had to stop. The fruit they expected to ship were still a bright, and very unripe, green.

All the lemons had been grown under similar conditions. A variable had changed after the lemons were harvested, and the lemons had failed to ripen. What could it have been? Look for clues in this chapter to the mystery of the still-green lemons. Then, solve the mystery.