The magic bridge

"Watch."
Jane dropped the apple over the side of the bridge.  John stood on his tiptoes and craned his neck to watch it fall.  Plop! It dropped into the water and then bobbed to the surface.  The current started to sweep it under the bridge, and they both ran to the other side to watch it reappear.  It did so swiftly, bouncing along on the wavelets... then it stopped... and reversed its path, passing back under the bridge, seemingly against the current.  The children ran to the first side of the bridge again to see it reappear, make a wide circle, three twirls and then resume its way with the flow, to disppear down the river.
"What do you think that was about?" Jane asked.  "I can't even see any whirlpools or anything."
"It was a fish," John replied confidently.  "It grabbed the apple and dragged it back."
"No it wasn't," insisted Jane.  "We would have seen it."
"Magic?" asked John, with some uncertainty.
"That is not a scientific answer," Jane chided her brother, with a smile to take the sting out.
"It might be," John replied.  "After all, if there is something going on here we don't understand, it may as well be magic, right?"
Jane thought about this for a moment, then grinned.
"You're right.  From now on, this is the magic bridge."
They both laughed and ran away to find a new game to play.
Meanwhile, under the bridge the troll was grumbling.
"Another apple drops from the sky, and my fishing line breaks again!"