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someone once told me it's good to put a pinch of salt into koolaid/other drinks in the summer when you're sweating.
That's to prevent hyponatremia--basically OVER hydration. If you are drinking *alot* of water, and sweating *alot*, you can run the danger of flushing all the sodium out of your system, which can be very dangerous (runners often see these tiny white crystals forming on their skin after a long run--that's the sodium being sweated out). Luckily, this is very rare and easily preventable. Most sports drinks, like Gatorade, contain enough sodium to replenish your supply. Or you can even eat a quick pretzel. But basically, your are only in danger of this if you are working out for long periods of time, especially in hot weather. If you are just puttering around in the garden, you don't need to worry.
As for the headaches, I get them sometimes too after a run. It usually means I'm either overheated, or dehyrdrated. Lots of water, followed by a cool shower (and maybe some ibuprofen) usually does the trick.