English: An example, given by Albert Einstein, which shows that a set of privileged frames exist in classical physics and special relativity, namely inertial frames. The elastic sphere and spheroid are distant enough so their interaction can be ignored, and their only motion relative to each other is rotation around the line connecting their centers. The frame where the sphere is not rotating is privileged because its physics do not require external causes. In general relativity, even inertial frames are no longer privileged.