Not many of us can open a 20-ton door, but the ancient pyramids of Giza were so perfectly balanced that it was not impossible. Many pyramids were equipped with massive doors that were not easily visible from the outside.
From the inside, they could be opened with minimal effort – even with just one hand. These doors were only discovered when the Khufu Pyramid was explored from within, and to this day, nobody knows how the stones were cut and balanced to make the mechanism work precisely.