A control sphere was a device created by the Great Intelligence to control its Robot Yetis. It was a glass sphere that was fitted into the chest of a Yeti and served as the brain. It emitted a beeping noise.
The Intelligence was able to control the Yeti and provide its motive power. Should the sphere be removed from the chest, it would move by itself to locate a deactivated Yeti and install itself into its chest. If the chest was blocked by something like a rock, the sphere would no longer think it is needed.