It is said that if you add photons into a box and close it, the weight of the box will be increased. My question is: How? Photons are understood to be massless, which is what allows them to travel at c. But, when photons are absorbed by a material body, they do add mass to that body. Now, when photons are in a box, but not absorbed by it, they are still lone photons, which are supposed to be massless. How then do they contribute to a net force downward by the box on the scale? (My understanding is that photons have mass equivalence in their energy by E=mc^2 [which makes them contribute to spacetime curvature], but that they do not actually individually have inertial mass.) The only way I can figure it is that the light in the box follows spacetime curvature, making there be statistically more radiation pressure on the bottom of the box than on the other sides -- assuming the insides of the box are 100% reflective.