Professor Pip knelt on the riverbank in the golden afternoon light, patting his bulging green lab-coat pockets. "Hmmm, where did I put it… ah!" He pulled out a small wooden tray, a handful of little twigs, and a paper cup full of water.

Mia and Leo crowded around. So did their friend Asha, who had come along for the day.

"Today," said Professor Pip, his ringed tail curling behind him, "we're going to learn how a chubby, furry, paddle-tailed rodent saves an entire train station from flooding."

"What rodent?!" said Leo. "A beaver!" said Pip, his eyes twinkling. "Four hundred years ago in Britain, people hunted every single beaver until there were none left. None! For four hundred years — no British beavers. Then, just a few years ago, scientists started bringing them back."

He set up the tray on a flat stone. "Watch what happens when heavy rain falls on land that has no place to hold it." He poured the cup of water straight across the empty tray. Whoosh! The water raced across and spilled off the edge, splashing the grass.

"That's what was happening to the Greenford Tube station in West London," Pip said. "Every big rainstorm — whoosh — water everywhere. Sandbags lined the corridors."

He began laying twigs across the middle of the tray, piling them up. He pushed in some little pebbles. "But in 2023, conservationists let a family of five beavers loose nearby. The beavers got busy. They dragged willow branches and built a wall across a creek." He patted the twig wall in the tray.

Then he poured the water again. This time, it slowed. Pooled. Spread out. Plip-plop. Only a tiny trickle dripped off the edge.

"Oh!" gasped Mia. "It's like a sponge!"

"Exactly right!" cried Professor Pip, clapping his paws. "Scientists call it a wetland. The beaver dam holds the water back, and a soggy, mossy area soaks it up — like a giant sponge. Then it lets the water out slowly, slowly, slowly. No more whoosh. The Tube station stopped flooding!"

He pulled out his pocket notebook. "And here's a wonderful bit: the beavers also brought back life. Freshwater shrimp came back. Eight new kinds of birds. Two kinds of bats. Even tiny brown hairstreak butterflies — gone for years — laid their eggs on the new branches."

He looked across the pond, where a real beaver dam glinted in the late sun. "One family of beavers. A whole world of life."

He handed each child a stick. "Now you try. Build the dam up higher — see if you can stop all the water?"