2 other numbers I read today on HN that made me look at things differently, although I knew them in other units but those didn't 'click' as much: air (including

2 other numbers I read today on HN that made me look at things differently, although I knew them in other units but those didn't 'click' as much: air (including the water in it) weighs about 4 kg/m3 (much more than I would have intuited), and the whole US uses 'only' 1.3 km3 of water per day.