It is not an easy task to keep a swimming pool so clear and clean. If you just set a pan of water outdoors in the middle of summer, you'll see that it ends up containing gunk very quickly. People have to resort to both chemical and physical means to keep the water clean and safe for swimming. The water is continuously pumped through a filter to trap particles, like all those bugs that fall in. To keep algae and dangerous bacteria from growing, chemicals such as chlorine are added. Chlorine also is added to your drinking water to keep those bacteria out of your stomach.