In a park, there are two friends, Emily and David, playing catch with their dog, Luna. They have a red ball and a blue ball. Emily throws the red ball, and Luna runs after it. While Luna is away, David hides the blue ball behind a tree. Luna comes back with the red ball but accidentally drops it into a nearby pond. A stranger walking by picks up the blue ball from behind the tree and continues walking. Unaware of these events, where do Emily and David think the blue ball is?

Respond very briefly.
Emily and David think the blue ball is still behind the tree where David hid it. David hid the blue ball while Luna was retrieving the red ball, so unless he told Emily or she saw him do it, they both have the last known location of the blue ball as behind the tree. Since the stranger picked up the blue ball without their knowledge, they have no reason to believe it is anywhere else but where David left it. Luna dropping the red ball into the pond is a separate event and does not affect their knowledge about the location of the blue ball.