She refuses to share.
At playdates, your child won't let anyone near his toys, but he'll scream "Mine!" and snatch away his friend's playthings.
How to help: Around 15 months, kids begin to develop a sense of their own identity. When another child takes one of his toys, he can feel as if he's losing a part of himself, says Bette Alkazian, a parent and a family coach in Thousand Oaks, California. Taking turns is the easiest way to introduce sharing: Have your child and his friend each play with the prized toy for a minute to reassure him that he's not giving it up forever. Supply things that encourage cooperative play, like a ball or blocks. If all else fails, try distracting him with another toy or activity. And don't forget to model sharing at home. If your child is hogging crackers, split some with his older sister: When he gives you one, smile, say "Thank you!" and then share yours. If you make it look like a game, your little one will want to join in on the fun.