S3·E10 Mirror Mirror

Nick and Sally are in the treehouse making funny faces at themselves in the mirror when they notice a bird flying back and forth into the tree branches. They discover that the bird is building a nest and they want to get a closer look but can't do it without disturbing the birds. The Cat takes them to a mirror world where they learn how reflections work and they concoct a kind of periscope that allows them to watch the bird without disturbing it.