Entering the interior of Khufu's pyramid is a separate ticketed experience beyond the general Giza admission. Only 300 people per day are permitted inside; tickets are sold at the main entrance gate from opening time and typically sell out within the first hour. If you want to go inside, arrive as early as possible — ideally at or before the 8 am opening.
The experience is unlike any other: you crouch through a narrow low passage (the "robbers' tunnel" cut in antiquity to the original ascending corridor), then follow the original ascending passage, growing taller as you go, until it opens into the Grand Gallery — a corbelled stone passage 46 metres long and 8 metres high, built with a precision that still baffles engineers. At the top, the King's Chamber is a plain granite room containing an empty lidless sarcophagus. The air is close and the walls bear four millennia of visitors' scratched names. The whole passage system is narrow, steep, and hot. Claustrophobics should skip it; everyone else should not.