Dehydration

.Mild to moderate dehydration is likely to cause:
Dry, sticky mouth
Sleepiness or tiredness — children are likely to be less active than usual
Thirst
Decreased urine output — no wet diapers for three hours for infants and eight hours or more without urination for older children and teens
Few or no tears when crying
Dry skin
Headache
Constipation
Dizziness or lightheadedness
 

Unfortunately, thirst isn't always a reliable gauge of the body's need for water, especially in children and older adults. A better indicator is the color of your urine: Clear or light-colored urine means you're well hydrated, whereas a dark yellow or amber color usually signals dehydration.

Treat children and older adults with greater caution. Call your family doctor right away if your loved one:

Develops severe diarrhea, with or without vomiting or fever
Has bloody stool
Has had moderate diarrhea for three days or more
Can't keep down fluids
Is irritable or disoriented and much sleepier or less active than usual
Has any of the signs or symptoms of mild or moderate dehydration
 
You can help prevent dehydration from becoming severe by carefully monitoring someone who is sick and giving fluids, such as an oral rehydration solution (Pedialyte, others) at the first sign of diarrhea, vomiting or fever and by encouraging children to drink plenty of water before, during and after exercise