You are camping in a remote region. The next day, you plan to take a long hike and return to camp before dusk. A friend suggests that after hiking, you might want to enjoy a warm shower. She suggests that before you head out in the morning, you should suspend a strong, waterproof, black bag full of about 5 gallons of water, so that it sits in sunshine all day. When you return, the water should be warm, and you can enjoy a luxury because of good planning. The next day when you return from your hike, you enjoy a warm shower at your campsite! The process that you used to get a warm shower uses .