[Tell It Right]

Natural gas is lighter than air.

Good point. But carbon monoxide is heavier than most air and odorless.

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Deep thoughts, by Jack Handey.

[HartleyMBaldwin]

Well, CO is actually just slightly lighter than air at equal temperature and pressure, but it’s close enough that the two tend to mix rather than stratify, even without agitation.

[nonliberal]

Or just use Sarin gas.