Our bodies are made up of at least 50 percent water. While we can survive for a few weeks without food, humans can only go a few days without water. Without enough water, dehydration sets in and causes dangerous drops in blood volume. But the reverse of dehydration is equally dangerous - too much water has led to the deaths of people before.
How does our body know when we're thirsty? A region of the brain called the subfornical organ can sense when we need water. It also responds to temperature too - that's why ice-cold water can quench thirst much quicker than lukewarm or hot liquids. Unlike hunger, thirst can be satiated in an instant with hydration. Scientists think this is one way for the body to protect us against the dangerous effects of too much water.