Hah, I remember my first CO2 sniff. I thought the scent coming out of the airlock was great so when I popped the lid I stuck my nose in and inhaled deeply... and then literally almost passed out.
I went a long time without repeating that mistake until I started kegging. I forgot that I left them full of CO2 after pressure testing so after I popped the lid I stuck my nose in there to see if it smelled clean.

There's an old saying in Tennessee — I know it's in Texas, probably in Tennessee — that says, fool me once, shame on — shame on you. Fool me — you can't get fooled again.
