Why lunar eclipses do not happen on every new and full moon?

Ans. Eclipses do not happen at the new and full moon every month because for most of its orbit, the Moon is above or below the ecliptic plane, i.e., the plane of Earth's orbit around the Sun. Only when the Moon's orbit crosses through the ecliptic plane, the Sun, Moon and Earth align to produce an eclipse. This occurs at least twice a year, at varying times, for a period of a few weeks.