Since I was a kid, I've thought I was "prone to migraines", and ascribed various triggers to them - sun exposure, heat, physical exertion, mental exertion, etc. I'd get a migraine sometimes after a long hike on a weekend - and also a long business meeting entirely indoors in an air-conditioned space.
Only when I was around 35, did I figure something out. All these situations lead to me getting dehydrated without any obvious accompanying feeling of thirst. Hiking all day will do it - walking around an outdoor shopping mall on a hot afternoon - or sitting in an all-day business meeting focused on the work at hand and forgetting to drink. And all these situations lead to a migraine - my only "migraine" trigger is simple dehydration, nothing more complicated.
The weird thing is, it took me a long time (decades) to put this together, because I just figured that I couldn't be dehydrated if I wasn't thirsty, and I had no association between "feeling thirsty" and getting a migraine.
I get what I consider normally thirsty in other circumstances, but somehow there's a failure mode where my body doesn't warn me. So now I just remember to chug lots of water (and electrolytes) if I'm exerting myself even if I don't really feel thirsty, and I can systematically avoid triggering migraines.
Now that I understand it the association is quite clear and obvious in retrospect.
Sports drinks are basically the same thing, but with excess sugars for “energy” (and weight gain).
When my wife was ill a few years ago the doctor suggested Angel Delight[1] to help maintain fluids. Until then it hadn't occurred to me you're still effectively drinking half a pint of milk when you eat a bowl.
[1] It's an instant dessert / mousse that you mix up with milk. https://en.wikipedia.org/wiki/Angel_Delight