If you start noticing that you're getting dehydrated then you should drink some water or other hydrating liquids to help overcome the dehydration.
Make sure you drink water or other liquids before you get thirsty to keep your body from becoming dehydrated and make sure that you drink more water during hot weather as you lose a lot of water through sweat.
If you notice severe dehydration or notice someone getting severely dehydrated you need medical attention right away so either get to the emergency room or call 911.
If the person becomes unresponsive and is dehydrated call 911 because being dehydrated can cause someone to not be responsive when you try to wake them up.
Other signs that someone is severely dehydrated are fainting, sunken eyes, rapid heartbeat, rapid breathing, dizzy feeling, very dry skin, pale skin, dark yellow pee or not peeing at all and Sleepiness, lack of energy, confusion or irritability.
If you notice or experience any of those signs you should get to the ER to get treated because if you don't get hydrated properly again you or that person could die.