The top flap on the big box came down and Simon felt the need to run for his life. River saw Simon bolt and looked like she was thinking, "What did I miss?"
After I fixed the box flap, Simon decided that the whole set-up required a safety inspection. It took him a few minutes to army crawl up to the boxes before deeming them fit to play in again.
River, completely unphased by the box flap drama, immediately hopped up onto the small box to survey her kingdom. Simon, ever the cautious one, kept circling the boxes to be sure that they didn't eat his sister.