In a simple yo-yo, the end of the string is fixed to the axis between the disks. If you wrap the string around the axis and then drop the yo-yo, the string will come undone and the yo-yo will spin as it falls. Once the string is fully stretched, it can no longer fall any further, but the yo-yo will continue to spin due to inertia, so it will wrap the string in the opposite direction and climb up.

Size: Width 5.4cm x Depth 5.4cm x Height 4.5cm