Aha! Captured this female Wool Carder bee Anthidium manicatum making a fiber ball from a lambs ear.
Good timing for me, although the image isn't that great.
The bee landed on the lambs ear, which wasn't so unusual, since the wool carders are constantly monitoring this patch.
But when she crawled under the leaf, I sensed that this was a female who was going to scrape fibers for her nest, from the underside of the leaf.
These bees are fairly quick when they scrape off the hairs of the lambs ears, and this seems to always happen on the underside of the leaves.
This means that to get a photo of the bees in action is always difficult, because there are always other leaves blocking part of the view.
Then when the bees fly away to their nests, grabbing onto the cottony ball, they are equally quick.
Here the bee is scraping the fibers from the leaf.