Staying hydrated while you hike is extremely important. When you become dehydrated from too little water you get a build-up of lactic acid in your muscles, especially in your legs. Your body begins to tighten up and hurt, and you'll get exhausted in no time. This can be particularly dangerous when you're out hiking with no access to fresh, clean water. Dehydration during hikes can also cause hypothermia, which is a dangerous raising of the body's internal temperature. This can cause nausea, lethargy, heat stroke, heat exhaustion, and more.
To stay hydrated during your hike, bring a few aluminum water bottles filled with fresh water. Whenever you feel a bit tired, or if you've been walking uphill for a while, stop and take a few sips of water. Don't drink too much at once or you'll give yourself cramps. Instead, just take a couple of drinks to make the thirst fade. If you don't want to pack water bottles, try a hydration pack. These are small backpacks that are designed to hold only water, usually with a way to drink the water while you walk. Carrying it on your back will also distribute the weight and keep you from becoming exhausted.
When you're hiking be on the lookout for signs of dehydration. These include thirst of course, by also sticky saliva, lost elasticity in your skin, and dark yellow urine. If you're hiking and become severely dehydrated without a fresh water source you can purify water you come across with a water purifier, although these can be very expensive. They'll kill the bacteria in the water and can make a good back-up plan to have if you're going to be hiking for a long time or going far off a path.
Staying hydrated while you hike will keep nausea, heat stroke, and lethargy at bay, as well as fight the exhaustion and muscle ache. Proper hydration will also save your life if you're hiking in the summer in high temperatures by fighting hypothermia. Never go hiking without bringing a large supply of fresh water with you, and ideally a backup plan like a water purifier.