There is a seeming paradox that is often mentioned in relativity, called the twin paradox. In the twin paradox, a set of twins does an experiment. One stays on Earth while the other flies off to a distant star at very high speeds and returns. According to relativity, the twin who went to the star and back will return much younger than his stay-at-home sibling. Paradoxically, the twin on the spaceship could also claim to have aged more because he can consider himself to be stationary.