The Cat in the Hat Knows a Lot About That! S01 E19 Night Lights

Nick and Sally are making shadow puppets with the Cat in the Hat but the batteries in the flashlight have gone out... they jump into the Thingamajigger and shrink down to visit the fireflies that live in the backyard. They learn that the fireflies communicate by flashing their lights. When the fireflies provide the light the kids need, the shadow puppet show can continue as expected.