As you're out hiking with your cat, a lettuce, and a vacuum cleaner, you come upon a river blocking the path. On the side of the river, there's a boat that's just big enough for you and one other passenger.
The cat can't be left alone with the lettuce.
The vacuum can't be left alone with the cat.
I take the cat across.
Then return.
Then I take the vaccum across.
I return with the cat.
I take the lettuce across.
Then return.
Pick up the cat, and cross a final time.