Three missionaries and three cannibals are on one side of the river, wanting to get across.
Unfortunately, the only boat available can hold a maximum of two people. The missionaries, wanting to stay safe, can never be on a side with more cannibals than missionaries (even for a moment!). The boat cannot travel under its own power, so there must be at least one person on board for it to cross.
How can the missionaries get safely across?
OK, I agree that Sam & Charlie's solution doen't need hand cuffs,
and works even if you consider the persons in the boat to still be a
threat to persons on the shore before the boat shoves off (which I
didn't assume).
However, most cannibal tribes and most missions have more than three
members. If we are willing to consider the persons in the boat to
no longer be a threat to persons on the shore, then using a pair of
handcuffs we can handle n missionaries and n cannibals, for all
positive values of n. The handcuffs, of course, prevent the
handcuffed cannibal from rejoining the shore party and overpowering the
missionaries.
In fact, handcuffs allow us to handle n+1 cannibals and n missionaries,
as long as we can somehow convince the cannibal to handcuff himself
before the missionaries arrive, and then to stay in the boat after all
the missionaries cross. And as long as the missionaries have all
of the keys. As Charlie correctly points out, the problem does
not require the cannibals to all cross.
Handcuffs still useful
