Faster than you can dress your 18-month-old, she can pull off all but her button-up-the-back shirt. "Fifty degrees is a bit chilly for your birthday suit," you tell her, starting over as cheerfully as you can, though you're already 10 minutes late. With little regard for weather or decorum, many 1- to 2-year-olds are fascinated with undressing.

The Naked Truth: The sensual pleasure of air against skin may be enticement enough, but often there's more going on. "A child this age is having fun with her growing competence," says Michael Jellinek, professor of psychiatry and pediatrics at Harvard Medical School. Kids love to practice new skills, and once they get good at one, they like to do it over and over. Pulling her arms out of her sleeves or slipping off her leggings are among the things a toddler this age can master.She's also intrigued by the act of doing and undoing: opening and closing a door, dropping and picking up (or having you pick up) a toy, putting on and taking off a pot lid, having you dress her and then undressing herself.

How long this phase lasts depends on why the child does it and how long it takes before she's ready to tackle another skill - like putting on her clothes. For some kids, says Donald Schiff, professor of pediatrics at the University of Colorado School of Medicine, the fascination is over in a month or two; for others, a year.

Addressing Undressing: When her antics present practical problems, distraction usually works much better than a simple no. If your toddler starts tugging on her tights at a party, for example, pick her up to go see who's at the door. Another solution may be a button-back top, one-piece rompers or other tough-to-undo clothes. (But bear in mind that they may complicate diaper changes.) Make sure the clothes you dress your toddler in are comfortable - not overly fussy or warm - or you'll give her added incentive to disrobe. Indulge her when you can, too, with opportunities to undress constructively, such as before a bath. Allow her to run around the house naked once in a while, and when the weather allows, let her relish the warmth in nothing but her sunscreen.