Endangered Animals River Crossing

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Move three endangered animals and their specialist rangers from the old reserve to the new sanctuary using a two-seat raft. Click animals or rangers on the bank where the raft is moored to board; press Cross river to sail. An animal cannot be left on a bank with another species’ ranger unless its own ranger is also there — the rule is checked when the raft crosses, not while boarding.

This puzzle is a conservation-themed version of the classic river crossing logic problem — the same family of puzzles as the missionaries and cannibals problem, studied in computer science as an example of state-space search. Each configuration of who is on which bank is a state; each boarding or crossing is a move to a new state. Invalid states are pruned by the constraint rules. With three species, the minimum solution requires eleven crossings. The puzzle is ideal for teaching systematic reasoning, tree diagrams and algorithmic thinking at KS3 — without any arithmetic at all.