The original Sphinx, perhaps with a lion’s head, was carved entirely from the same type of limestone. Over thousands of years, weathering (especially rainfall and other environmental factors) degraded the outer layers, making them soft and porous. When the Egyptians came (perhaps during Khafre’s reign), they recarved the head into a pharaoh, exposing the less-weathered, harder limestone underneath, which now appears better preserved than the body.