3. Does the Earth go around the sun, or does the sun go around the Earth? Both and neither. Whether we answer one way or the other depends on our choice of reference frame. For most practical purposes, it's probably easiest to regard the Earth as fixed and to describe the motions of the Sun, Moon, planets and fixed stars in the sky. (Despite Copernicus, the Sun is still seen to rise every morning.) Physically we can say that both the Sun and the Earth go around their common center of gravity. (Given the differences in masses, I suppose that point might be inside the Sun, but not precisely at its geometrical center.) The Earth-Sun system in turn is moving around the center of our galaxy, which itself is moving in the larger universe. 6. Lasers work by focusing sound waves. True or false? Acoustical lasers exist. http://en.wikipedia.org/wiki/Sound_amplification_by_stimulated_emission_of_radiation I guess that technically, 'LASER' stands for 'light amplification by stimulated emission of radiation', so the sound analogue of that device should be called a 'SASER', but the term 'acoustical laser' is also used.