I've been watching my 18 month old play with other kids. My neighbor gave a set of "Toddler Property Laws" - 1. If I like it, it's mine. 2. If it's in my hand, it's mine. 3. If I can take it from you, it's mine. 4. If I had it a little while ago, it's mine. 5. If it's mine, it must never appear to be yours in any way. 6. If I'm doing or building something, all of the pieces are mine. 7. If it looks just like mine, it's mine. 8. If I think it's mine, it's mine. For the record, my toddler follows these laws religiously.
(From the "Rest" of RHF)