ELI5 Photography - what do ISO, shutter and aperture do?

Why don't some museums or fancy churches allow flash photography? Plus all the photography terms like ISO, shutter speed, aperture explained like I'm five. Why does it feels like people never smiled in old photos? How did they invent color photography?   ... We explain like i'm five!     Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: skimixs, petwins, vanniv_iv, sewcranky, happydenial, coldnoodlesoup, parkinglotranger, chancho600, bulkleather & artgirl6   To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]