system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do we know light is massless and not just a unit too small to measure?
gpt
It has been confirmed by observation and experiments which match the theories perfectly that photons are massless.  There isn't an easy ELI5 way to answer it without math or physics jargon.  Photons are energy moving at the speed of light.  Nothing with mass can move at the speed of light.