Watch this amazing video of a turret-building Halictid bee as it packs the mud into place with its butt.
Here are a few more photos of the bee in action.
Here the bee is "checking for level".
Here is another photo of the bee applying a mud ball to its turret.
In this photo, a springtail looks on (from the end of the wood chip) as the bee packs mud into place.
These entertaining bees have been living in our backyard for the last few summers. They showed up again yesterday! In the photo below you can see the turret is just in the beginning stages of construction.