Echo, a repeated sound. The term is usually used to refer to sound that is reflected from a surface and returned to its source after a noticeable delay. A person can typically hear his or her own echo where a hard surface, such as the side of cliff, reflects sound waves directly back to the person. The use of echoes to locate objects is called echolocation.
A black hole occurs when a massive star dies -- its enormous mass implodes and becomes so heavy that it bends space. So how do astronomers detect something that they can't see?
The man immortalized on the left was behind the three laws of motion and the universal law of gravitation. He was also competitive, temperamental and fascinated with alchemy. How well do you know Newton?