It may be the hardest thing for toddlers and preschoolers to do. No, we're not talking about potty-training. The dreaded s-word - sharing - can incite tantrums like no other. But the earlier you start teaching and modeling sharing, the better your child will become at sharing. Start helping your child understand the concept by buying just one of his favorite treat that's easily shareable - M&Ms, French fries, whatever it might be, dole out the item, allowing the child to take what he wants. Then next time, give the child the container and ask if he can share with you. Take notice not only when playing results in conflict, but also when it's going well. Praise kids for playing well together, saying something like, "It's nice to see you two having fun and playing so nicely."
Age 2 is when kids get a sense of possession, and that's when you can start using language to express the concept of sharing, even if they don't quite understand yet. With siblings forced to share something that's divisible - say, the last cookie or a pile of Legos - it may help to allow one child to divide the item with the understanding that the other child gets to pick his share of the goodies first. Therefore, it's in the dividing child's best interest to make things fair. At age 3 or so, kids begin to understand that if it's a toy, then the item will be returned. It may help to time turn-taking and give each child a minute or two with the toy, then switch off. On the other hand, it's important - especially when there's sibling rivalry - to allow kids not to share. When you know a younger brother will just step in and mess up his older sister's Barbie collection, then it's OK to intervene and have the kids play separately. Finding activities for them both to share, like playing musical instruments or making forts in the living room, are ways to foster togetherness without the need to share.