It doesn't matter which man you ask. The answer you get will tell you
where the million $ is, although you still won't know whether the man
you asked was the liar or the truther. (But who cares about that. Right ?)
Pick either 'guard',and ask him:
"<em> IF</em> I were to ask you 'Is the money behind your door ?', would you say 'Yes' ? "
Answer:
a wave in which the direction of displacement is the same as the direction of propagation, as a sound wave.
