words = ReadList["unixdict.txt", Word];

wordLadder[words_, start_, target_] := Module[{wordSelection, graph, path, noSolution = "impossible"},
    If[StringLength[start] != StringLength[target], Return[noSolution]];
    wordSelection = Select[words, StringLength[#] == StringLength[start]&];
    graph = UndirectedEdge @@@ Select[Subsets[wordSelection, {2}], HammingDistance @@ # == 1 &];
    If[! SubsetQ[VertexList[graph], {start, target}], Return[noSolution]];
    path = FindShortestPath[graph, start, target];
    If[path == {}, Return[noSolution]];
    path];

tests = {{"boy", "man"}, {"girl", "lady"}, {"john", "jane"}, {"child", "adult"}};
Map[{#[[1]] <> " -> " <> #[[2]] <> ": ", wordLadder[words, #[[1]], #[[2]]]}&, tests] //
    TableForm[#, TableDepth -> 2]&