One sunny morning, Feluś decided to build something unusual on the playground. From his backpack he pulled out a pile of colourful socks that his mum had forgotten to put in a drawer. - Hey, Wanda! - he called out to the squirrel, who was just trying to bury a peanut under the swing. - Do you want to build a sock castle? Wanda jumped up and down with joy. Together they started weaving fantastic towers out of socks with dots, stripes and hearts. Instead of a bridge, there was a sock rope across the sandpit, and on top of the castle hung a sock with the image of a cat. When Feluś wanted to open the gate to drive the car inside, suddenly the sock castle started to shake gently. Something rustled under the checkered tower and a pink tail protruded from a hole in the yellow sock. Feluś and Wanda exchanged glances, holding the zipper so it wouldn't fall apart. - Do you hear that? - whispered Wanda. - Who's hiding in our castle over there?