Since we've all been sick forever, we have a large number of ear thermometer probe covers lying around the house. They look like this:
Boo has observed me pumping breastmilk for Mudbug with my Medela pump, whose breast shield looks like this:
He walked up to me with two probe covers stuck to his chest and announced that he was going to pump milk for Mudbug too. Such an act of love has never been shown by a big brother for a baby sister, I think.