A、They become clogged before losing effectiveness. B、They’re equipped with built-in measuring devices. C、Their safety, limits ar

Kennedy: I’ve just found the most useful device at the new out-door store--a water purifier.
Jackie: A water purifier? You want to carry even more equipment with you on our hiking trip?
Kennedy: If it’s light weight and saves work, yes! And this one fits that description.
Jackie: Really? I’d bet it takes up a lot of room.
Kennedy: Not at all. It’s a straw, much like the one you use to drink a soda water or a milkshake.
Jackie: Really? That is small.
Kennedy: Yeah, it, s plastic and has a series of mineral and chemical filters that strains harmful microbes from water.
Jackie: Yes, but how good a job can it do filtering the germs out of stagnant pond water or water from lakes and streams.
Kennedy: It’s fantastic. It makes the water entirely germ-free.
Jackie: But what about salt or chemical pollutants that might be in the water? Does it clean those out?
Kennedy: No, it doesn’t remove salt and chemicals. But those are really not major concerns when we’re out hiking and camping.
Jackie: Mm…How long will a straw last?
Kennedy: 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 is reached and the straw stops drawing water.
Jackie: I think I’ll go to get one for my camping trip this weekend. It will surely be carrying a lot of water or having to boil it at the campsite.