> My son has an elementary school homework assignment where he's supposed to do
> simple observations on a solid, liquid, and gas for seven days.  Solids: easy,
> liquids: seven is probably exact the number that is easy to name.  Gases?  So
> far, we've done air, boiling water, and the smoke from cooking fish.  But
> that's about all I can think of without either getting dangerous or overly
> egg-headed.

Double-pane windows contain sealed gas (argon, usually) instead of air,
to avoid internal condensation.

Radon can come into a house (and it's dangerous), coming from the ground.

Fabien.

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