https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#Composite_systems https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalenc... gives some hints

https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#Composite_systems https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalenc... gives some hints: > A simple example of an object with moving parts but zero total momentum is a container of gas. In this case, the mass of the container is given by its total energy [...] Similarly, even photons, if trapped in an isolated container, would contribute their energy to the mass of the container. Such extra mass, in theory, could be weighed in the same way as any other type of rest mass, even though individually photons have no rest mass. i.e., it's a consequence of energy-mass equivalence, which applies equally to isolated systems as it does individual particles.