[double nitpick] Steam is also invisible, what you commonly see is condensed droplets of water, not steam
Superheated steam is incredibly nasty, If you are ever sent into a boiler room to look for a steam leak (even a tiny one) make sure you wave a large spanner in front of yourself 'cos it's invisible it's going to chop your fingers off [/double nitpick]

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'The eye can only see what the mind is prepared to accept'