A River-Crossing Puzzle

I have solved too many puzzles in my life. When I see a new one, my solution is always the intended one. But my students invent other ideas from time to time and teach me to think creatively. For example, I gave them this puzzle:

Two boys wish to cross a river, but there is a single boat that can take only one boy at a time. The boat cannot return on its own; there are no ropes or similar tricks; yet both boys manage to cross the river. How?

Here is what my inventive students came up with:

  • There was another person on the other side of the river who brought the boat back.
  • There was a bridge.
  • The boys can swim.
  • They just wanted to cross the river and come back, so they did it in turns.

And here is my standard solution: They started on different sides of the river.

I gave a talk about thinking inside and outside the box at the Gathering for Gardner conference. I mentioned this puzzle and the inventiveness of my students. After the conference a guy approached me with another answer which is now my favorite:

  • They wait until the river freezes over and walk to the other side.

One Comment

  1. Georgiy Kuznetsov:

    One boy was in the boat rowing, another one
    was swimming along holding to the stern.
    It takes little or no buoyancy to support
    a swimmer’s face above the water.