Pointing at the white cloud rising from a kettle, Om says, "Look, that's steam." His sister says…

Why

Om is calling the white plume 'steam' because that is what everyone calls it, but water as a gas — true steam — has no colour and cannot be seen at all, just like the water vapour already in the air around you. When the hot invisible gas leaves the spout and hits cooler air, it loses heat and its molecules clump back together into tiny liquid droplets — a mist, the same as fog or a cloud. That mist is what you see. Test it: look closely right at the spout of a boiling kettle — there is a small clear gap between the spout and where the white cloud begins; that clear gap is the real, invisible steam, still too hot to have turned back into droplets. His sister is right: the white cloud is tiny drops of liquid water; the actual gas, true steam, is invisible.