If your child ends up with an achy ear a few days after swimming in the lake, chances are she's got swimmer's ear -- an infection of the ear canal or outer ear (different from the middle ear infections that kids sometimes get along with a cold virus). Here's what happens: Having a lot of moisture in the ear breaks down the skin's natural defenses against bacteria and fungi, which then take hold and grow. Kids who have particularly dry skin (due to eczema, for example) are especially susceptible to swimmer's ear, says Max April, M.D., professor of clinical otorhinolaryngology at Weill Cornell Medical College in New York City. The symptoms include pain in one ear (the ear may even be tender to the touch or swollen and red) and itching or a feeling of fullness.

How to treat it: Ease the ache with an age-appropriate dose of ibuprofen. A warm washcloth held against the ear can also help. If the pain becomes unbearable, your child develops a fever, or she has cloudy drainage from her ear, call the doctor. He may prescribe antibiotic drops.

How to prevent it: Dry your child's ears thoroughly after she takes a dip. Have her tilt her head to one side and then the other to drain out all the water, then sop it up with a towel (never a cotton swab). If your little mermaid gets swimmer's ear a lot, try earplugs.