is a player down if his knee is on another player?

Question
Is a player who has the ball considered down if his knee is resting on another player who is on the ground?  Is the other player the same as the ground?

Answer
No on both questions.  A player could be sitting on another person hypothetically and not be called down.  The only possible way it could be yes, I would think, is if referee feels forward progress has been stopped and blows the play dead. Great question.

Coach B