So, it was another dinner conversation. Somehow, for whatever reason, we brought up the question, "What is real, but invisible?"
Brian shouts, "I know! Air!" and immediately went on to talk about air and wind, etc. I could tell Honor wanted to join in, so I leaned over and whispered into her ear, "Sound." She threw that out with a big beaming smile and turned to me for more.
Brad and Brian continued to talk about how air might be visible.
Brian asked if the air could be blue, but still be invisible? We weren't sure. Dad asked, if it's invisible, how do you know it is blue? Brian also tried light, but of course, you can see light, you just can't get your hands on it.
Brad suggested that if you blew up a balloon, you could see the air. Both kids immediately disagreed. Dad wanted to know why not, because you could tell by the balloon's shape that air was in there. They couldn't explain, but I said it was because if the balloon was closed with the air inside it, you know it's there, but how could we see it? Might as well be invisible. Or even blue.
In the meantime, we were trying to explain how correct the different answers were, or if there were exceptions - for vibrations, we pointed out that you can see waves, and they are vibrations, as is light - sometimes you can see these vibrations, sometimes not. And we continued to try and come up with other things that are real, but that you cannot see, like thoughts....
Then Honor said, "I know! ... Earwax!"
Out of the mouths of babes. The conversation deteriorated from there. Brad pointed out that 'the back of your head' is real but you can't see it. Brian started going in depth about internal organs. I just did my best not to snort gravy, as I demonstrated another (correct!) answer to the original question: laughter - real, but invisible. (Laughing is visible. Laughter is sound, therefore invisible, mostly... )