Why doess the 4 moon phases appear to be either fully illuminated or not illuminated or partially illuminated?

"From any location on the Earth, the Moon appears to be a circular disk which, at any specific time, is illuminated to some degree by direct sunlight. Like the Earth, the Moon is a sphere which is always half illuminated by the Sun, but as the Moon orbits the Earth we get to see more or less of the illuminated half."

Information from: http://aa.usno.navy.mil/faq/docs/moon_phases.php

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