When Jack was four months old or so, we left him with his grandfather for an afternoon as a test run for my upcoming return to work. Andrew and I used the baby-free time to go find some bedside tables at the unfinished furniture store. We found some perfect ones and bought them along with some stain and poly. The man who helped us pick out the poly made very clear that we were not to shake it when it was time to mix it, as that would introduce bubbles. He even wrote "STIR" in black Sharpie on the lid so that we wouldn't forget.
And so we didn't shake it. Any time we rearranged the four trillion paint cans we had, we made sure to pick that one up and set it down again without shaking it. We were very careful.
So when I finally did stain the first of the bedside tables a few months ago , I knew there would be no bubbles in the poly. And when I opened the can, I found out I was right! There were no bubbles at all! It would have been really easy to see them if there were, because bubbles show up very plainly in solidified polyurethane.