My son (who is three) doesn't miss anything, so he's been aware that I've been having trouble breastfeeding his new sister. Last weekend when I was still having some success nursing (now I'm just pumping all the time and feeding her breastmilk from a bottle), my son was in the other room with his Grandma while I fed the baby.

He shouted from his room: "Mama, are you feeding the baby?"
Me: "Yes!"
Him: "Is she eating from your armpit?"

Apparently my son needs a little anatomy lesson. I wasn't exactly sure how to answer that, so I said, "Sort of!" and he thought that was good enough.