"Will grow to 600 times its original size!" My kids love these toys. If you put the toy in water (or any liquid), the toy's special material will absorb and retain huge amounts of liquid and so will expand in size.
The material used is a “superabsorbent polymer”—and the Consumer Product Safety Commission recently cautioned parents that this material warrants further investigation because it may pose a potential safety hazard to children.
What is the concern?
These toys are colorful balls made of superabsorbent polymers. Small initially, they will grow when exposed to water—or to the fluids inside a child’s or a pet’s body. Such toys can then swell to a significantly larger size inside the body, potentially leading to an airway obstruction or to a blockage in the stomach and intestines.
The U.S. Consumer Product Safety Commission warned in December 2012 of a recall of these toys that was prompted by a serious health emergency. A child in Texas ingested one of the balls and the ball swelled and blocked her small intestine, requiring an emergent operation.
What should you do?
Avoid toys that contain these superabsorbent polymers.“Will rapidly expand in water” or other similar claims should serve as a red flag. These toys (like the balls mentioned above) are frequently colorful and can look like candy to small children.
Supervise your child or children if they are using any toys made from superabsorbent polymers.
Be aware that these polymers can be found in many other household products. They are, in fact, what makes disposable diapers so superabsorbent. They're also found in those water-retention pellets for gardening.