Maybe this scenario is familiar: You are at your child's weekly playgroup, watching your child play alongside another child of about the same age. Your little one is playing with a particular toy that the other child is eyeing. Inevitably, the other child reaches for it, grabs on tight, and pulls it away. Your child lets out an angry squeal and looks to you for help. You and the other parent jump in. One of you offers, "Let's try to share."
These children are still what developmental experts refer to as "egocentric." The children honestly believe that the world revolves around them and are incapable of taking another perspective. So, unfortunately, their diplomatic skills have not been honed, and they do not understand how to share.
That doesn't mean that it is a waste of time for you to explain to your child that she'll have to wait to use a toy or tell her why it is not acceptable to take a toy from another child. Children learn through experience and interactions with others. Right now, it is their job to pursue their interests and explore the world. This is accomplished through play.
The first way your baby played, before she became the mobile wonder that she is now, was through her observations of others. This happened every time you propped her in a bouncy seat in front of another person (including you). Even though she was not playing directly with the other children or playing with the toys in the same manner, she was actively participating though her observations.
Soon after children learn to sit and become more adept with their fingers, they engage in the next kind of play known as parallel play. An example of this type of play is the scenario described above. Even though they are not playing with each other, sitting side by side, these children study each other's interactions with toys, verbalize (to themselves), and learn from each other's ideas.
Similar to the scene above, if a girl is playing with a truck, chances are another child will want to play with the same truck. The trick is for parents to learn the delicate balance between supporting their play and preventing the oh-so-common altercations over a toy. One way is to make sure that when you host a playgroup, you have more than one of particular types of toys (such as trucks, balls, and puzzles) available.
It is important not to rush your child through this type of play. Children naturally move out of parallel play with the foundation for successful cooperative play and the ability to begin understanding the concept of sharing and negotiating. (Learn more ways to encourage play, here.)