Analysis: Since you can only bring one thing across the river at a time, you can only bring a sheep across the river for the first time. You can bring a wolf or a sheep for the second time, but you can't put it over there, so you came back. It's enough to bring the sheep back at the first time, send another one over the third time, and finally bring the sheep over.

Explanation: The first time I took the sheep across the river, the wolf and the cabbage were left;

The second time I took the cabbage across the river, the wolf was left, but when I came back, I had to put the sheep back together. Bring it back;

Take the wolf across the river for the third time, leaving the sheep;

Finally bring the sheep across the river.

One *** needs to be taken four times to complete.