I was recently reading a book to a group of preschoolers in which the animal characters were fighting over a toy. We talked about the problem these animals were having and I asked the kids if they thought sharing was difficult. They all enthusiastically agreed that sharing is really hard!
These children know from experience how challenging it is to share. When they play together there is often an argument over who can play with which toys. For example, Max had two cars - one in each hand and Tommy wanted one of the cars. Max quickly put both his hands behind his back refusing to give up either car.
I've found that in situations like this one of most effective responses is to describe the problem and then ask the kids how they are going to solve it. I explained "I see a problem. Max has two cars and Tommy would like to have a car too. How do you think you can solve this?"
Simply pointing out the issue typically causes the children to stop, look at me and start thinking about what's going on. Often these creative preschoolers come up with their own unique solutions. By leaving it in their hands, they are increasing their problem solving skills while also becoming more aware of the feelings of others.