I know that we aerate the wort to ferment. I've read that when priming and bottling, we should avoid the introduction of air as much as possible into the bottling bucket. Where does the oxygen come from that the yeast requires to carbonate the bottles? I would expect that when fermenting with an air lock that there would be little, if any entrained oxygen left in the beer at the end of fermentation. What am I missing? I'm about to bottle my first 12 gallon batch.