At the end of a narrow street, behind Tosia's big house, an old pear tree grew. One night, when the sky was full of stars, Tosia saw something unusual through the window: a glowing point fell straight from the sky and disappeared into the grass under the tree! "Was it a star pebble?" whispered Tosia. The next morning she called her best friend, the little robot Piko. Together they decided to find the mysterious pebble. They put on their space helmets and pretended their garden was Planet Green and the old pear tree was a space rocket. When they approached the tree, they saw something shiny between the leaves. But right next to it was a tiny entrance to a burrow in the ground, and from inside came a quiet squeaking sound... Tosia looked at Piko and whispered, "Maybe someone has already found our pebble?" The robot flashed its lights too. It was very curious. Tosia knelt down by the burrow and...