system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Not sure how ELI5 this can get, but how is that two atoms of hydrogen and one atom of oxygen combine into something (water) that's heavier than air even though they're lighter than air separately?
gpt
> water is heavier than air

Oh but that's the typical riddle: which one is **heavier**, 1 Kg of water or 1 Kg of air? Answer: naturally, they weight exactly the same, 1Kg. 

What to want to look at is not the *weight* but the weight *per unity of volume*. While it's true that one molecule of water (H2O) is lighter than one molecule of oxigen (O2), 1 meter cube of liquid waters packs way more water molecules than there are air molecules (oxigen and others) in 1 meter cube of air. That's because in air there's much more empty space between molecules, as it's typically the case with any gas. This makes the cubic meter of air much much lighter than the cubic meter of water.