He was having trouble with the Twin Paradox. If one twin remains on Earth while the other travels near the speed of light for a decade, upon returning, the astronaut twin would be younger than the stationary twin due to relativistic time dilation. But then again, velocity is relative. So relative to the moving twin's ship, it's the Earth that has been traveling near the speed of light. So it's the stationary twin who should experience time dilation with respect to the moving one. He was sure that he was missing something here, but he didn't know what. He was too embarrassed to ask the friendly neighborhood relativity expert.