From your profile, your child is 1 year old?

Sharing is a very difficult concept especially if you only have one child at home. My son has been in preschool since he turned 2 years old and I can tell you that the kids in the school do not trully learn to share until they are about 2 1/2 years old.

So don't despair! You'll just have to keep repeating that it's OK for other kids to play with his toys because they'll give them back. Or go the route of taking turns.. this is usually an easier concept.

Good luck!

I think it all depends on how old he is! It's a pretty weird concept for them if they're quite little.

My nearly 2 year old understand the taking of turns, but playing together with the one toy is still pretty much a scary and alien concept!!!

try this... let your kid experience the same situation. Let him realize how others feel when not being shared with. It's a bit harsh to think, letting your kid suffer, but I have to say, experience is the best teacher... especially for kids.

How old is he?

We played "The sharing game" with both our boys so they understood the word firstly before we started to try to force the behaviour.  This is how we did it:

When he's in a good playful mood, tell him "we're going to play the sharing game."  Grab a ball and sit opposite each other (with our first boy, he had no problems sharing with Daddy, but wouldn't share anything with me, so we made it a threesome for him) and roll, pass or throw the ball to each other.  Start off by saying "(son's name) share with Mummy, now Mummy is sharing with Daddy" etc etc.  And after each time he passes the ball say "yay!  That's great sharing!" and really praise it up.

Then the next time you are telling him he has to share, he will understand that he will eventually get the toy back.  Also, say things about taking turns, because again, that reinforces to him that he's not loosing the toy forever, he will get another toy (doesn't work if it's food though cause he ain't getting that back)

Our younger one is very unhappy about having to share, and will always put up a fight first, but he gets the message that a tantrum won't change the fact that he has to share.  He's had priviledges like toys and sweets taken away because he won't share them.  But he learns and does share a fair bit, and we make sure we praise him when he does.