What are these little mud nests on the ceiling of your garage?

Mud dauber nests. Mud daubers are a type of solitary wasp that build small, mud nests on ceilings, walls, and other sheltered surfaces. They construct these nests by gathering mud and water and then shaping it into a small, vase-like structure. Each nest contains several cells, each of which houses a single egg. The female mud dauber will provision each cell with several spiders or caterpillars, which serve as food for the developing wasp larvae. Once the larvae hatch, they will consume the provisions and then spin a cocoon. The adult wasps will eventually emerge from the cocoons and leave the nest.