Baby input -- feeding by breast, bottle or spoon -- can be fun, but almost no one enjoys baby output. Tykes dislike urine and feces next to their skin, and parents want the wastes contained, away from skirts and shirts, couches and carpets. The ideal diaper would protect both property and progeny. A waterproof cover stops some leaks, but liquid still could squeeze through the gap between diaper and baby. The diaper's inside layers need to absorb urine so it will stay put. Cotton diapers work on a simple principle -- provide lots of surface area to which water can stick. If you want to soak up a spill in your home, you probably reach for a sponge or a towel. Both have a large surface area with many small holes and fibers. When water wets part of a surface, it is attracted to neighboring parts. For example, water in a glass creeps a fraction of an inch up the sides because of an attraction between the water's molecules and those of the glass. This phenomenon causes water to creep so strongly into the many small holes of a sponge that even gravity is defeated. Dip one corner of a sponge into water, and the whole sponge eventually will be wet. The cotton in cloth diapers works like a sponge, as does the cellulose fuzz inside a disposable version. But sponges don't keep a strong grip on liquid. After a sponge absorbs a large amount of water, much of it can be squeezed out. Likewise, a diaper can absorb urine with just cottony fuzz or cloth, but when baby sits down, the urine can gush out again. Manufacturers of some disposable diapers found a way around this problem. They added small special particles to their cottony padding, creating a kind of roach motel for liquids: Urine checks in, but it doesn't check out. Instead, it soaks into the dry particles to form a softer but firm gel. A gel is a mixture of liquid and solid with some properties of each. The solid part typically is made of large molecules such as protein or synthetic polymers (plastics) in a sponge-like meshwork like that of felt. When the microscopic gaps in the meshwork are empty, the gel is shrunken and rigid, but when liquid creeps in, the gel swells and becomes softer and wobblier as the meshwork loosens. You've probably eaten gels in the form of Jell-O or Gummi Bears, products made from a solid protein called gelatin, with water in the gaps. Would powdered gelatin absorb water fast enough to make a good diaper? You can test this idea by trying to dissolve food gelatin, such as Jell-O brand or Knox, in cold water. But you won't see much; even after 24 hours, dried gelatin won't have soaked up much water. The package directions tell you to use hot water for a reason. The heat helps to dissolve the dried gelatin proteins, producing a gelatin-in-water solution that behaves mostly like water. But, as the solution cools, the microscopic strands of protein molecules stick together to form the wobbly scaffolding of a gel, with the water still in the gaps. Incidentally, warm urine won't dissolve the gelatin. The gel particles in disposable diapers are made of plastic, which contains synthetic polymers consisting of long chains of smaller molecules. Attached to those chains are sodium ions, the gel's secret weapon for luring water molecules inside its structure at a reasonably low temperature. The polymer gel in a dry diaper has only about 1 percent water, so the particles are very small. When the diaper gets wet, water molecules slip into the gaps between the polymer molecules to get closer to the sodium. The gel particles swell up. Given the strong chemical attraction of sodium for water and the strong molecular structure of the polymer, you can no more squeeze the urine out of a diaper gel than squeeze the water out of Jell-O. Diaper gels can hold about 100 times their dry weight in distilled water but only 50 to 60 times their weight in urine. This is because urine is slightly salty, and the sodium in salt competes with sodium in the gel for water molecules, pulling them from the polymers. Still, this is enough absorbency for about a teaspoon of gel particles to absorb a cup of urine without providing wet surprises for anyone picking up a crying baby. Paul Havlak is a computer science researcher at the University of Maryland and output engineer for his 6-month-old daughter. CAPTION: A Swell Gel The water-retention power of gels is amazing. Gels, as you may have learned in the article about disposable diapers, are jelly-like mixtures of solid and liquid with some of the properties of both. They are wobblier than most pure solids but firmer than pure liquids. In addition to their role in diapers, gels also are used to enhance the water-retention power of soil. Garden shops sell bags of the stuff that can be mixed into potting soil or even into topsoil. Just as in a diaper, the chunks soak up water and swell to many times their dry size, hanging on to water that might otherwise sink through porous soil. You can experiment with the water-holding power of gels. HERE'S HOW MATERIALS One or more high-absorbency disposable diapers. Although most brands contain similar gel particles, our tests show that they are easier to remove from Huggies and Toys R Us brands than from Pampers. A plastic bag large enough to hold one diaper. Scissors. Measuring spoons and a measuring cup. A glass or plastic container that holds at least one cup. Use one with a lid or cover with plastic wrap. Water. Table salt. METHODS 1. Cut into the diaper lining to find a fuzzy cottony material. This is cellulose embedded with pieces of gel, varying from powdery to grainy. Empty all of the cellulose and any loose particles into the plastic bag. Shred the cellulose in the bag to release the gel particles. Remove and discard the fuzzy cellulose, leaving as much of the powdery gel in the bag as possible. 2. Measure out 1/2 teaspoon of the powder. Put in a glass with 1/2 cup water. Seal with the lid or plastic to avoid spilling because the resulting goop can be messy. 3. Swirl to mix and let sit for a few minutes. What is the consistency of the mixture? 4. Try adding another cup of water. 5. Open the lid and add one tablespoon of salt. Seal and shake. How does it change? WHAT'S GOING ON? In an unused diaper, gel particles are very small, giving them a large surface area for their weight. This makes it easier for water to reach each small piece of gel and easier for water to soak all the way in. In the diaper, the gel pieces start almost completely dry and severely shrunken. But after soaking up water, they swell to huge proportions. If you found (as we did) that a half-teaspoon of particles held a cup of water, you proved that they swelled to at least 96 times their original volume. This is because there are 48 teaspoonfuls in a cup. Since the particles were not tightly packed in the spoon, they actually absorbed a bit more than 96 times their volume. Adding the salt should have made the gel particles shrink because sodium from the salt competes with sodium attached to the gel polymers. As explained in the diaper article, the attached sodium increases the gel's ability to take up water at room temperature. But sodiums from the salt pull some water out of the gel, making the mixture runny again. You can perform several variations of this experiment. For example, what is the gel like if you add only a little water, say a tablespoon, to a half-teaspoon of particles? What happens to the gel if you let it dry for several hours or days? And, of course, you can experiment with intact diapers, even comparing brands.