A pollywog is a young frog or toad in an early stage of life, before it grows legs and can hop on land. You might know this creature by its more common name: a tadpole. When a frog egg hatches in a pond or stream, out wiggles a tiny pollywog with a round body, no legs, and a long tail for swimming. It breathes through gills like a fish and spends all its time underwater, eating algae and tiny plants.

Over several weeks or months, something remarkable happens. The pollywog gradually transforms: back legs sprout first, then front legs appear, the tail slowly shrinks away, lungs develop for breathing air, and the mouth changes shape for catching insects. This amazing transformation is called metamorphosis. By the time it's complete, the pollywog has become a fully formed frog or toad ready to live on land.

Sailors also use pollywog as a nickname for someone crossing the equator for the first time, since they're new to that experience, just like a pollywog is new to being a frog.