When my students show me their work with a blue strip up the top and a green strip down the bottom I tend to get quite theatrical, stagger around, clutch my throat and hoarsely whisper "The Great Nothing!" I take them outside and ask them to show me this nothingness and after some discussion (their nothing generally seems to refer to the air about them) they start thinking about all the 'stuff' around them. Now all I have to do is just act up and they walk away to do more. I am not sure if they are actually learning something or if they are just too sick of my bad theatrics, but as they progress through the school they seem to have taken it on board.
Sometimes, I also give them round pieces of paper to just throw them a bit. I also do not like the sun icon. I spend a lot of time outdoors, specially as the weather heats up, showing them how to look.