Aha! Puzzle This
River Crossing

We start with a mother (m), two daughters (d1, d2), a father (f), two sons (s1, s2), a housemaid (h), and a dog (c - canine) on the west (W) shore, and they all want to get to the east (E) shore.
W = {m, d1, d2, f, s1, s2, h, c} // everyone on the west shore
E = // no one on the east shore

Let's move everyone over ...

housemaid and canine go east, and the housemaid comes back:
W = {m, d1, d2, f, s1, s2, h}
E = {c}

housemaid and s1 go east, h and c come back:
W = {m, d1, d2, f, s2, h, c}
E = {s1}

father and s2 go east, father comes back:
W = {m, d1, d2, f, h, c}
E = {s1, s2}

mother and father go east, mother comes back:
W = {m, d1, d2, h, c}
E = {f, s1, s2}

h and c go east, father comes back:
W = {m, d1, d2, f}
E = {s1, s2, h, c}

father and mother go east, mother comes back:
W = {m, d1, d2}
E = {f, s1, s2, h, c}

mother and d1 go east, housemaid and c come back:
W = {d2, h, c}
E = {m, d1, f, s1, s2}

h and d2 go east, h comes back:
W = {h, c}
E = {m, d1, d2, f, s1, s2}

h and c go east
W = E = {m, d1, d2, f, s1, s2, h, c}