You may wonder if you’ve been drinking enough water, especially when it’s hot out. There’s a lot of confusing advice out there about how much you really need. The truth is that most healthy bodies are very good at regulating water. Elderly people, young children and some special cases—like people taking certain medications—need to be a little more careful. Here’s what you need to know.

“Water is involved in all body processes,” says Dr. Jack M. Guralnik of NIH’s National Institute on Aging. “You need the proper amount for all those processes to work correctly.”

The body regulates how much water it keeps so it can maintain levels of the various minerals it needs to work properly. But every time you breathe out, sweat, urinate or have a bowel movement, you lose some fluid. When you lose fluid, your blood can become more concentrated. Healthy people compensate by releasing stores of water, mostly from muscles. And, of course, you get thirsty. That’s your body’s way of telling you it needs more water.

At a certain point, however, if you lose enough water, your body can’t compensate. Eventually, you can become dehydrated, meaning that your body doesn’t have enough fluid to work properly. “Basically, you’re drying out,” Guralnik says.

Any healthy person can become dehydrated on hot days, when you’ve been exercising hard or when you have a disease or condition like diarrhea, in which you can lose a lot of fluid very quickly. But dehydration is generally more of a problem in the elderly, who can have a decreased sensitivity to thirst, and very young children who can’t yet tell their parents when they’re thirsty.

How much water does your body need? Guralnik says you have to consider the circumstances. “If you’re active on a hot day, you need more water than if you’re sitting in an air-conditioned office,” he explains. An average person on an average day needs about 3 quarts of water a day. But if you’re out in the hot sun, you’ll need a lot more than that.

Signs of dehydration in adults are being thirsty, urinating less often than usual, having dark-colored urine, having dry skin, feeling tired or dizziness and fainting. Signs of dehydration in babies and young children include a dry mouth and tongue, crying without tears, no wet diapers for 3 hours or more, a high fever and being unusually sleepy or drowsy.

If you suspect dehydration, drink small amounts of water over a period of time. Taking too much all at once can overload your stomach and make you throw up. For people exercising in the heat and losing a lot of minerals in sweat, sports drinks can be helpful. But avoid any drinks that have caffeine.

Remember: the best way to deal with dehydration is to prevent it. Make sure to drink enough water in situations where you might become dehydrated. For those caring for small children or older people with conditions that can lead to dehydration, Guralnik advises, “You need to prompt the person to drink fluids and remind them often. It’s not just a one-time problem.”