When did she get so possessive? you wonder. What's going on?

It's something very normal. "Staking a claim on things is one way toddlers assert their growing sense of self," says Parenting contributing editor William Sears, M.D., author of The Baby Book.

The good news: You can make use of this developmental leap by introducing the concept of sharing. While you can't expect toddlers to initiate sharing, they can do it with your help and supervision, says Dr. Sears. The next time your child refuses to lend a toy to a playmate:

Stress teamwork. Suggest ways both kids can play with the item at the same time, recommends Paul Jose, Ph.D., a Chicago-based child psychologist. If your son and a pal are fighting over a plastic dump truck, say, "You fill up the truck with sand, and your friend can dump the sand out. You'll be a team!" Says Jose, "The point is to get your child excited about sharing."

When P.J. Kaminski, a mom from Pasadena, CA, sees her 2-year-old son, Andrew, not sharing his toy castle with a friend, she says, "Won't it be fun to show him how it works?" Andrew gets so caught up in showing that before he knows it, he's sharing.

Divide and conquer. Have the two children take turns playing with the coveted toy a few minutes at a time, suggests Dr. Sears. To avoid squabbles, set a timer so that each child knows exactly when his time is up.

Pull a switcheroo. If the other child has some of her own toys, let the two kids play with one of each other's toys for a few minutes, then switch, says Dr. Sears. Even though your toddler has to give up something, she's compensated for the loss with a new plaything to examine and explore until she can get hers back. Thanks to youngsters' short attention spans, one swap will probably be enough to satisfy both kids.

Let "mine" sometimes be fine. Certain things, such as a beloved blankie or teddy bear, are simply too cherished to be passed around to your child's playmates. You can avoid any possible conflict by putting these special items away when friends come over to play. As long as you encourage your child to be generous with her other possessions, says Jose, "she'll still get the message that sharing is an important skill to learn."