It’s a hot, summer day, the sun is shining and you can’t stop sweating. You realize you haven’t had much to drink all day.

You could be getting dehydrated. This happens when your body doesn’t have enough fluid.

“Think of what a house plant looks like that doesn’t have enough water,” said Leon Veldhuizen, M.D., a Marshfield Clinic Health System family medicine physician. “It starts to wilt and shrink up. It’s the same with our bodies. If we don’t have enough fluid, things won’t work properly.”

How we become dehydrated

We mainly lose water through perspiration, urination and excreting bowels. When it’s warmer, you lose more fluid through sweat, making it easier to become dehydrated. Elderly people and the very young are most affected. Illness or fever, vomiting and diarrhea (from infection or from a bowel disorder), and increased urination from uncontrolled diabetes or from certain medicines like water-pills, also can cause dehydration.

Young children can become fussier, sweat more, stop crying, become lethargic and have a decreased number of wet diapers or dark colored urine.

Dr. Veldhuizen says the most important thing is to prepare before going into a warm or hot environment so you don’t become dehydrated.

Learn what dehydration feels like

This graphic explains what happens when you become dehydrated and what you can do to stay hydrated.