'Crossing the River With Dogs' is a logic-based river-crossing puzzle based on the textbook of the same title. The idea of any river-crossing problem is simple: get all of the people or items from Side A to Side B. The problem-solving aspect comes into play when there is limited room on the 'boat', and the actors in the problem have various conditions that need to be met.
In this puzzle, there are four humans and four dogs. Each dog belongs with its respective human. There is a total of eight passengers, but only three can fit onto the boat at the same time. The dogs can get very anxious if they're left alone, and must either be with their human, or with other dogs, in order to feel at ease. For the boat, any human can pilot the boat, but dogs cannot. There is, however, one dog who received training at a special boarding school for dogs, where the trainers taught him how to drive a boat. He is the only dog in the group of dogs that can pilot the boat.
Rule #1: Dogs cannot be alone!
Any dog, even the dog who can drive the boat (who we'll call 'Zeus'), cannot be alone at any time. If a dog is alone on either side, you may not cross. Similarly, if a dog is alone on the boat, you may not cross.
In order to make sure a dog is not alone, one of two conditions must be met: The first is that dogs are okay if they're together. Two or more dogs are comfortable around each other. The second way to make sure the dogs are not alone is to make sure each dog with their human. For example, Human 1 and Dog 1 are a pair. If Dog 1 is with Human 1, he will be okay. However, if Dog 1 is with Human 2, they are
not a pair, and Dog 1 will feel alone and afraid. Remember, though, dogs are okay if they're with their human
or other dogs.
Rule #2: The boat must have a driver.
If the boat has no driver, it cannot cross the river. Because the boat can only hold three passengers, and there a total of eight passengers, multiple crossings will be necessary. For each crossing, the boat will need someone to drive. Any human can drive the boat. Humans are okay being alone (in this puzzle, at least) and can drive the boat without help. Zeus -- the dog who has been trained to drive boats -- needs a bit of assistance. He can drive, but he must meet the rule of either having other dogs with him, or having his human with him. Otherwise, he will be too anxious to cross the river.