自然にできる「丸い石」ってホントにあるの？
In some parts of the world, almost perfectly round stones can be found, and they were not made by machines. One famous example is the stone spheres of Costa Rica. These stones are between a few centimeters and 2 meters wide. Some scientists think that they were made hundreds of years ago, but no one knows exactly how. Some may have been shaped by nature, while others may have been made by people. Their round shape is still a mystery that researchers are trying to solve.
