Gus is obsessed with books. This high-energy, full force blur of a toddler will not stop for much, but he will sit still on my lap for book after book. He's been this way since very early on. When he was about 4 or 5 months old, he'd happily allow me to read him half a dozen to a dozen books at a time. As an introverted bookworm, I of course love this (except that it makes it hard for me to hold boundaries sometimes when he is asking me to read another book. Who wants to say "No" to that?) Also, it has been helpful when we need to get a concept across to him. Tails Are Not for Pulling is a popular one around here, and much appreciated by the cats!
His favorite books when he was wee (he still loves them though he's begun to add others to the repertoire) were Todd Parr books. I think all the bright colors really captured his attention when he was really tiny. Many of Todd's books talk about differences; being unique, different types of families, and adoption. And we always add commentary while reading ("our family has two moms", "you're adopted"). So now, when we get to a line in a book about adoption ("it's okay to be adopted", "some families adopt children"), Gus points to himself! (The first time he did this with Shelley, he pointed to himself and then gave her a huge hug!) I'm not entirely sure he understands exactly what being adopted means, but he knows he is. And he feels that it's a positive thing.
We have a bunch of friends who also have adopted kids, so I think Gus may have gotten a little bit confused about it once we started telling him that Malcolm is adopted, and Baby Ben is adopted, and Baby Vaughan is adopted, because then he started pointing at his baby doll and saying "adopted", so I suspect he might think that all children are adopted. But I'm not sure. All I know is that it feels pretty great that he knows. Even if the concept isn't entirely clear, we're laying the groundwork. We talk to him about his birthmom and his birth/adoption story. We strive for openness in all things in our family.
The other thing I'm pretty sure he knows, that I hope he knows, is that he is LOVED. I don't believe that love is enough to erase all the pain he may one day feel about being adopted. It's complicated, and his feelings about it are bound to be too. But I do hope that if he is secure in our love for him he will be able to one day talk about and explore the feelings that he has surrounding the loss of his biological family. And I hope, for now, being accepted, and being loved, and knowing he is adopted (whatever that might mean for him in his toddler brain at the moment) will be enough. I feel like it's a good start.