听力原文：W: I just found the most useful device at that new outdoor store, a water purifier. M: A water purifier? You want to carry even more equipment with you on our hiking trips? W: If it's lightweight and saves work, yes. And this one fits that description. M: Really? I bet it takes up a lot of room. W: Not at all. It's a straw. Much like the one you use to drink a soda or milkshake. M: Really? That is small. W: Ya. It's a plastic, and has a series of mineral and chemical filters that strain harmful microbes from water. M: Yes. But how good a job can it do filtering the germs out of stagnant pond water or water from lake and streams? W: It's fantastic. It makes the water entirely germ free. M: But, what about salt or chemical pollutants that maybe in the water. Does it clean those out? W: No, it doesn't remove salt and chemicals, but those are not really major concerns when we're out hiking and camping. M: Mm. How long will a straw last? W: For about 100 gallons, but don't worry about measuring that large amount of water. There is a built-in safety feature. The mineral and chemical filters become clogged long before the limit has reached and the straw stop drawing water. M: I think I'll get one for my camping trip this weekend. It'll sure be carrying a lot of water or having to boil it at the camp site. Where will the special straw probably be most useful?