So the Sims get a doll sent to them by their long-lost Great-Aunt when they are born, that the parent gets out of the mailbox and it goes directly into the baby's inventory.
When they become toddlers, they can play with it. And play with it they do!
And if they play with it enough, when they become children, you will get a popup message that the doll wants to come out and play, prompting you to take it out of the kid's inventory and place it into the world...
AND IT COMES TO LIFE.
It's your sim child's imaginary friend!
At first, only the child whose toy it is (was?) can see it, much like the Social Bunny of TS2... and when the child isn't home (at school, for example,) the imaginary friend turns back into a toy doll sitting on the floor that can be picked up and "talked through" like other toys of that nature. But when the child gets home, the toy comes to life again, and plays NON STOP with your sim child. NON STOP. Like, my kids were always in the yellow because they wouldn't stop to eat or sleep over playing with their imaginary friend.
Now... here's where it starts to get really creepy. (Like it wasn't before, right?)
As your sim ages, the friend ages too. So here, we have a teen and his imaginary friend, who is also a teen, all grown up and doing ummm... teen things.
*backs away slowly*
Oh, but it gets EVEN BETTER.
Sometime during your sim kid's childhood and/or teendom, he or she will receive an Opportunity... bring a Rainbow Gem to the Science Facility, and they might be able to make your imaginary friend a real part of the family!
Of course, this is optional; if you prefer the doll to stay imaginary and unplayable, I'm sure you can and I'm sure it will age to adult right along with your Sims (might make for some interesting times, eh?) but I went ahead and did the opportunity with one of my teens...
And Rags here turned into a real boy!! O_O
A very... real... boy. Just like all other real boys. Even though my sim used to cuddle him, beat him on the floor, and sleep with him in her crib as a toddler...
CREEPY MUCH? But you know... I love it!! I admit I was kind of hoping he would become playable as a doll-like form vs. a human, but still, it's great. :) I plan to have a few sims grow to adult without turning their imaginary friends human though... I want to see adult-sized imaginary friends! (I do hope they exist!)
More later... I have to tell you all about the baby lawn gnomes sometime. :D