I love mud dauber wasp nests.
Each cell, at one time, had a single egg and was packed solid with tasty spiders for the baby mud daubers to grow on. They grew up alone in the dark, pupated, and then hatched out and flew away into a world they had never seen before.
This one is a little unusual, in that it only had one tenant. Typically they are re-used by solitary bees. I found it in a shed, and I guess our little bee buddies didn’t look there.