Water is made of hydrogen and oxygen molecules and exists in solid, liquid, and gas forms on Earth. Most of the planet's water is located in oceans, while the rest is found in lakes, rivers, ice caps, clouds, underground aquifers, and sea ice. Humans need water for many functions like carrying nutrients, lubricating joints, cooling the body, and uses in industry and agriculture. While people generally need 6-8 glasses of water per day, athletes and those who are ill may need more. Water shortages can occur when demand exceeds supply, and conservation is important since less than 1% of the world's water is available for human use.