Carbon dating is performed on once-living matter. Thanks to cosmic rays, the carbon in the atmosphere is radioactive at above-equilibrium levels. Once fixed int

Carbon dating is performed on once-living matter. Thanks to cosmic rays, the carbon in the atmosphere is radioactive at above-equilibrium levels. Once fixed into a plant, it decays. That makes it possible to measure the time since the carbon was last in the atmosphere. Plant matter made its way into the pyramids both in the stuff that was put inside them, and also in the mortar, which was baked over wood fires.