From Scientific American:

Introduction
When we think about diapers we usually think about babies. But did you know that astronauts also have to wear diapers sometimes? Astronaut “diapers” are called maximum absorbency garments (MAGs), and astronauts wear them when they have to stay sealed in their spacesuits for long periods of time, such as during spacewalks or when their spacecraft leave orbit to reenter the atmosphere to return home.

For babies and astronauts, the most important thing for a diaper is to prevent leaks. But did you ever wonder how a disposable diaper actually works? What’s inside the diaper that allows it to absorb all that wet stuff without making a mess? In this activity we’re going to explore the substance in diapers that allows them to stay leak-free—in a cradle and in outer space!

Background
Whether it is made for a baby or an astronaut, the major disposable diaper brands all contain a powdery chemical absorbent called sodium polyacrylate, which can absorb over 300 times its weight in water! That’s about equivalent to a normal adult being able to suddenly absorb the weight of a blue whale, about 40,000 pounds!

Being able to absorb a lot of liquid is what makes sodium polyacrylate important in diapers. The chemical absorbs the liquid in the diaper, pulling it away from the skin. The diapers that astronauts wear can absorb two liters of water—the same amount of liquid that’s in a big soda bottle at the grocery store! And thank goodness that they can absorb this much, because nobody wants to have a wet diaper while they’re fixing a spaceship!

In this activity you’re going to separate sodium polyacrylate from diapers, and test its absorbency (how much liquid it can hold) for yourself. One way you will measure this is by measuring volume, which means the amount of space …

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