Our outside area includes a mud pit. It is exactly what you think it is: a sunken area filled with mud.
There was a small problem with our mud pit. Last summer the adjacent patio was removed when the deck was replaced. This left a depressed area with nothing to keep the mud in the pit. The weather warmed up, we began using water, and the mud began moving to parts of the yard where we (I) didn't want the mud to go.
For a quick fix, I moved one of the logs to that space. I knew it wouldn't hold back all of the mud, but it would slow it down until I could put in something more permanent.
The children noticed a problem with this almost immediately.
With the new log in place to block mud and water, they set to work making a giant puddle. Which worked well for a few minutes, but the water began to seep under the log.
The first attempt to block the water involved layering rocks. That slowed the water, but they didn't have the puddle they wanted. The next time we met, they used both mud and rocks to block the passage. This worked much better, but there was still come leaking. The third attempt blocked all of the water from getting through. The main difference in this attempt was that there was time for the mud to dry.
Follow up research included watching an episode of Peep and the Big Wide World. There are a few episodes that have to do with building dams and understanding how water moves. This led to discussions about beavers and how they make dams.