You know those little foam things, that you put it in water and it grows to a hundred times its size? Well, this one starts out looking like a worm, but then it grows into the shape of a fish.
It would be perfect to put on the end of a kid’s fishing pole so they can “go fishing” in any water, and they’ll catch something every time.
Of course, the drawback is they have to leave it in the water long enough to grow, but still, I think it’d be neat. They could be sold with a little fishing pole, or separately as refills, and could grow into different kinds of fish.