The trickiest concept is water vapour. I spoke about steam coming out of the kettle, and the little gap between the spout and the steam where the steam is 'invisible'. That, I said, is water vapour. The water is in the air and you can't see it. I went on to explain how there is water vapour all around, but we can't see it. This water vapour rises high up into the air and when it cools down, it turns into clouds and then it rains.
Now just recently it had been very dry (probably a week or so without rain). I said to Matthew that I wished it would rain. He said,
"Should I go and turn the kettle on, Mummy?"