Why does the moon change shape?

You ask: Why moon change shape?

Moon not change. Moon always round rock, like fire-lit boulder in dark. Sun always shine on one half of moon — half bright, half dark, same as Caveman face by fire. Moon walk circle around Earth, take about one moon-cycle — twenty-nine and half days, science tribe count. As moon walk, Caveman see lit half from different side. Moon between Caveman and sun — Caveman see dark side, no moon. Moon on far side — Caveman see full lit face, big round moon. In between, Caveman see slice of lit part — that the banana moon. Not Earth shadow doing this — shadow only for eclipse, rare thing. Cave wisdom: moon not shrink and grow. Caveman just see more or less of moon's sunny side.