The answer to the riddle "What goes up but never goes down?" is "age." Age is a measure of the elapsed time since a person's birth. As time can only progress and cannot be reversed, age must only increase.Continue Reading
Even in extreme cases, such as a person moving at speeds near the speed of light, age does not decrease. A person moving at relativistic speeds ages more slowly than a person moving at a slower rate, but she does not become younger.
In Einstein's Twin Paradox, the parents of twin 10-year-old boys keep one son on Earth and send the other on a round trip to a star 25 light years away. When the traveling twin returns 50 years later, he has only aged six months, making him 49.5 years younger than his twin brother.Learn more about Puzzles & Brainteasers