"Oh, no, lightning starting fires is just because everything combusts - or melts - when you put enough energy in. And brain damage...yes, it is mostly lesions. But the proper functioning of the brain involves little teeny tiny micro- lightning-bolts jumping from cell to cell. ...There's, like, chemicals that mediate that, but the lightningbolts are the cool shit. Pardon my language."
"Humans are not mostly made of blood, though, blood, being a circulatory fluid, is just the easiest thing to extract from a human. The internals of most cells? Water. There was a funny little story about that actually, however did it go...
"The beginning of the evolutionary record was all water-breathing creatures, for values of 'breathing' that include 'the ocean just goes along however it likes'.
"And then, evolution, the process of - traits that are more fit to survive the stresses of an environment being statistically - not individually, you can be the best example of your species ever and still get struck by lightning - more likely to be present in future generations of an organism -
"Well, I'm unduly personifying it, to say it 'came up with' anything, because it's a blind idiot god - er, well, not even a god at all, nobody worships it, but like, philosophically speaking - but - it came up with this glorious idea of keeping the fluid it used to move nutrients and signals from point A to point B, inside a sealed system, so it was harder to fuck up the homeostasis - er, the...not dying of heatstroke or poison or suchlike -
"Anyway. The primitive analogues of blood vessels are still pumping saltwater around, because this is the ocean and that's what you have on hand. ...I'm not even going to think about plants. Anyway. Blood vessels."
"Next, the many-many-times-removed ancestors of today's life start venturing out of their saltwatery homes into freshwater ones, and dying because they exploded. ...That's a real thing; I think I could probably rig up a demonstration of osmotic pressure from relatively common supplies. Also to do with why your fingers prune if you're too long in a bath, I believe.
"Anyway - where was I going with that...
"Evolution, presumably.
"Right. Anyway. It invents 'pumping extra water out of the cell'. ...Which is why urination exists, but I'm not going to get into actual discussion of the mechanics there because I hardly know them.
"And then these horrible dumbass creatures stop being satisfied with being in water at all and start flopping around on land, fins into limbs and whatnot - you can see the similarities in skeletal structure - but at no point in this process has it been easier to stop using water anywhere. It's called the universal solvent for a reason.
"Er. Might not be called that here. Still does take most things into solution. There's no true universal solvent, though. Not without magic bullshit.
"...Pardon my language again.
"Anyway!
"...What was the point of this even -
"Ah, right. Water, and how prevalent it is.
"...Actually, y'know what, let's start from the opposite end - a survivalist's adage.
"I think it was once said that you can go three minutes without air, three days without water, and three weeks without food - as a rule of thumb, exceptions apply, etcetera etcetera.
"Now consider how much fluid the average human needs to replenish per day.
"It's a significant fraction of their body mass!
"...So yeah. Water. It's everywhere."