If droplets form outside a cold bottle, what is the source of that water?

Correct answer: Water vapour from the air outside the bottle

Direct answer: A, water vapour from the air outside the bottle. The cold bottle surface cools the thin layer of surrounding air touching it. Air can hold water vapour, although the amount it can hold depends on temperature. When this moist air is cooled to its dew point, it becomes saturated. Further cooling makes some water vapour change into liquid water. This change from gas to liquid is called condensation, and the liquid appears as droplets on the outside surface. The water does not pass through sound glass, and no plastic melting is involved. A is correct because the droplets come from atmospheric moisture. B is wrong because water cannot normally leak through intact glass. C is wrong because oxygen is a gas and is not converted into these water droplets. D is wrong because melting plastic cannot produce ordinary water droplets. Memory cue: cold surface plus moist air equals condensation; the outside water was already present as invisible vapour.