[math-fun] Wordbox challenge

The following puzzle has been floating around work. It's been called "Word Box" but I've seen with a little googling that that name is used for other kinds of puzzles. In any case here it is: You're given a list of words, and an m by n grid. You want to assign letters to the points on the grid so that you can spell out each of the words by starting at some point in the labeled grid, and tracing a path where each step can go left, right, up or down from the current point. The explicit challenge is the names of the nine (this was before Pluto was demoted) planets. Give word boxes which are 4 x 8, 5 x 7 and 6 x 6, and (much much harder) show that you can't do it for 4 x 7 and 5 x 6. Victor