They are known as Water Balz, and at first glance they look like marbles: small and spherical, brightly colored. But these balls are made of superabsorbent polymer, and they are capable of rapidly expanding to the size of a racquetball when doused in water.
This unusual feature has made Water Balz a popular toy. But it is also the very feature that has prompted some pediatricians to warn parents that the toys can pose a unique hazard to small children.
