I have a very intelligent three (almost four) year old who has a mixed relationship with thunder and lightning (see an [older question](https://parenting.stackexchange.com/questions/12672/fear-interest-in-thunder-and-lightning) from me, for example).  He likes seeing it, but is also afraid of both.  Part of this stems from probable sensory disorder - loud noises bother him much more than most children; but I think some of it is fear, as well (as he fears it even when it's not happening currently), in particular fear of the *unknown*.

One of the ways I'm trying to get over this, in addition to that from the other question, is to help him understand what lightning and thunder are in a physical sense - ie, an approachable but accurate scientific description.  I want to help him understand what lightning and thunder are, what is _dangerous_ about them, and what is _not dangerous_ about them; and why we are (mostly) safe from both while indoors.  In general, I explain physical phenomena as completely as possible, given his age; for example, I explain that rain comes from the sky when clouds are too full of water (which is true, to a limited extent).  I won't explain barometric pressure yet, but cold/hot air masses I might explain at some point soon.

How can I explain this to a preschooler, so that he will have as complete of an understanding as possible at his age, and in particular focusing on having a good idea (in his head) of what they are so they're not as scary?