One thing to consider: you can't actually show water in the gaseous state. Steam is invisible/transparent. So is gaseous water vapor. Many people see condensed water droplets (clouds, contrails, or the wisps of vapor we sometimes call steam) and think that's gaseous water, but it's not -- it's the liquid state in the form of fine droplets.

I'm not sure if that's too fine a distinction for your fourth graders.

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"It is quite clear that Occam's razor does not sharpen in your pyramid." (Nicolas)

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