Expandable water toys, also called Grow Monsters, are those cheap little plastic-like toys that get bigger after you leave them in water. They come in all shapes and sizes, with popular varieties including dinosaurs, reptiles, insects, robots, and chicken eggs. Instructions vary, but most of the toys need to be kept in a cup of water for a few days in order to roughly quadruple in size.

While it may seem like magic, their growth is rooted in chemistry. The toys are made of superabsorbent polymers that suck in and retain a large amount of liquid proportionate to their mass. 

The superabsorbent polymer technology required to make Grow Monsters originated back in the early 1960s, when scientists at the USDA were trying to develop a polymer that would help soil conserve water. Using starch-grafting—a process of joining acrylonitrile polymer onto the backbone of starch molecules—and adding water, they found that the polymer allowed the soil to absorb more than 400 times its weight. The USDA introduced the polymer, called Super Slurper, to scientists at chemical companies to experiment with the technology. Scientists at Dow Chemical then expanded upon the USDA’s work, discovering sodium polyacrylate, a superabsorbent polymer that can absorb up to 300 times its mass in water.

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In the 1970s and 1980s, this superabsorbent polymer technology evolved from an agricultural application to something entirely different. Companies in the U.S., Japan, and Europe began selling disposable personal hygiene products made with sodium polyacrylate. Proctor & Gamble and Kimberly-Clark put superabsorbent polymers in their menstrual pads, tampons, and diapers so they would retain more liquid than plain old cotton or sponge could. Since then, hygiene companies have moved away from using polyacrylates, partly because of the health risk of toxic shock syndrome in some users.

Over the years, various brands of expandable water toys have been recalled due to a different health risk—choking. A 2012 article in Pediatrics, the American Academy of Pediatrics’ journal, discusses their potential dangers. These toys are more dangerous than regular toys that children might accidentally swallow because, as they increase in size, they can get stuck and obstruct the child’s intestines. An 8-month-old baby swallowed a marble-sized “Water Balz” and needed surgery to get the ball out, since such toys can expand to the size of a racquetball when mixed with water (and other liquids in the digestive tract).

Despite product recalls and health risks, many Asian companies continue to manufacture and sell expandable water toys, which today can be made with a mixture of sodium polyacrylate, polyvinyl acetate (a polymer used as a binder) and ethylene vinyl acetate (a polymer used to mold the toy into whatever shape it’s supposed to be). This superabsorbent polymer technology also has many other uses: The polymers are used in wound dressings and surgical pads to absorb blood, food packaging to absorb excess moisture, and artificial snow.

So just how much will your expandable water toy grow? It depends on how long you keep it submerged in water, but the water itself also matters. Interestingly, the difference in the ionic concentration of tap water versus distilled water reportedly means that a toy in distilled water can absorb as much as 10 times more water than the same toy in tap water, meaning that your tiny dinosaur could get even terrifyingly bigger than you expected.