This is a question I got for my class.
We have the following situation. We take two tuning forks, we hit them, then we take one of the tuning forks and walk away with it. The sound changes as we walk away and it isn't consistent anymore.
EDIT: I see I was a bit unclear, tho the question itself is unclear since i don't know anything except that. Silly teacher.
So, to clarify, two forks(my guess is that they have the same frequency). We hit each fork at the same time, one student is holding the fork #1 next to the blackboard, other student is walking away with fork #2.