3 monks and 3 demons want to cross a river

Why

Krish is correct that a small boat and a strict safety rule make this hard — but 'hard' is not the same as 'impossible'; it just means the crossing needs a longer, more careful sequence than the simpler fox-chicken-grain puzzle. The key insight is that demons can safely shuttle the boat back and forth by themselves (demons alone are never a danger to anyone), so the plan sends pairs of demons across early to clear space, then carefully lets monks cross only when the numbers on both banks stay safe — including one clever step partway through where two monks cross together and then one monk and one demon come back, which temporarily looks like it undoes progress, but actually re-balances both banks so the remaining monks can cross safely afterward. Test the very first few moves: send 2 demons across (3 monks + 1 demon left behind is safe, monks are not outnumbered; 2 demons alone on the far side is fine too); send 1 demon back; send the last 2 demons across (3 monks alone on the near side, 3 demons alone on the far side — both fully safe) — already 4 people have made progress in just 3 trips, using the demons' freedom to move safely as the engine of the whole solution. His sister is right: with demons doing most of the shuttling and monks only crossing when the count stays safe on both banks, everyone gets across — in the standard solution, in exactly 11 boat trips.