I find them stuffed into the toes of a new pair of sneakers. I find them wedged into a sheaf of seaweed snacks. I find them in the over-inflated bag that contains my new inhaler, and in the vacuum-sealed one puckered around my kids’ 3D printing filament. “DO NOT EAT,” they all admonish me, and I find myself slipping them under the top layer of the garbage already in the trash can, as if my kids wouldn’t be able to control their urge to taste whatever is inside these tiny white pouches. Silica gel packets are everywhere, their presence seemingly the only thing keeping our packaged food crispy and our belongings free of mildew. How on earth did they all get here? Is silica gel taking over the world?

Tear its little Tyvek wrapping, and spill a packet of glassy silica gel beads into the palm of your hand; they won’t hurt you. They are made of the same stuff as sand: “Silica” means “silicon dioxide,” which is the primary component of most drinkware, windshields, and the screen of whatever electronic device you’re reading this on. But glass has a density of around 2500 kilograms per cubic meter, and crystalline silicon dioxide (quartz) is around 2650. Silica gel, on the other hand, is more like 700 kilograms per cubic meter. It may look fully dense, but in fact it’s shot through with countless tiny pores. If your windowpane is like a thin sheet of solid ice, then a silica gel bead is like a tiny snowball.

Zoom in on a silica gel bead with a scanning electron microscope, and its smooth surface turns discontinuous, riddled with voids about 2.5 nanometers across (roughly the diameter of a strand of DNA). This microstructure gives silica gel radical properties. The silica gel packets in my kids’ seaweed snacks are just a little bit bigger than postage stamps, and have a total mass of about a gram. That single gram of silica gel could have an internal surface area of eight hundred square meters—the size of almost two basketball courts.

These factors allow silica gel to adsorb up to 40% of its own weight in water vapor, through a process called capillary condensation. When humid air migrates into the pores of a silica gel bead, its vapor pressure increases, causing water to condense onto the silica gel’s internal surfaces. At the risk of anthropomorphizing water, it’s as if it prefers to be a liquid, stuck within the silica gel’s tiny capillaries, than a vapor, carried along with whatever else is in the air.

If you find a packet of silica gel in an imported snack or the pocket of a new jacket, it’s probably there to filter water vapor out of air. Like most filters, there is a limit to how much water a piece of silica gel can hold; its internal surfaces are finite in size, and as a result there is a finite amount of humidity that a given packet of silica gel can adsorb. Luckily, silica gel vendors offer nifty calculators so that their customers can size their silica gel packets to the volumes of air they wish to dry out—and how dry they want the air to be. If you wanted to desiccate the air inside of a child’s balloon, you’d calculate its volume (let’s call it 14 liters), then make some assumptions about its temperature and humidity (around 35°C, 75% RH), then