Dog Crossing

Five people (A, B, C, D and Lisa) and five dogs (a, b, c, d, and Lisa's dog) are on a hiking trip. Each person owns one dog (the letters match). They come to a river and want to cross it. A rubber raft will hold up to three at a time, any combination of dogs and humans. Only the five people and Lisa's dog can drive the raft. How do you get them across the river? The catch: a dog cannot be in the presence of any humans unless its owner is also there. [Addendum: assume everyone exits the raft after each trip.]

Source: A tutored student of John Knoderer's of on newsgroup rec.puzzles 9/14/97. I was later told it is related to an old Henry Dudeny puzzle.

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