Mouse soup is usually only a winter problem because that is when liquid is harder to find and when the water buckets are inside the barn up against the walls. Thirsty mice climb up the barn walls and jump into the buckets because they smell/sense the water is not frozen. Fortunately, mouse soup is preventable by floating a raft or sticking a stick down into the water so that the mice have some method by which they can get out of the water.
* I'm guessing that the mouse was still alive due to a combination of heated water (electric bucket) and timing (he hadn't been swimming to exhaustion yet).
