where do they get these things?

I know kids pick things up from anywhere and everywhere, but this latest one Alex has picked up I can't figure out where he got it but it is so damn cute. SO he went to see my mom last night and was playing with my little sister and my mom, and was running from my mom and said "na-a-na-a boo boo" even used it in the teasing context! I didn't teach him this and neiter did Cameron, My mom claims she is not responsible, so maybe from daycare? I will ask today. It just makes me laugh. On top of that he has started just saying your name to get your attention and then nothing else, well last night my little sister Jamie (10) was the subject of this and Alex wondered around my moms saying "Jamie" constantly, and jamie had to ask my mom to try and get him to stop cause it was annoying her.. Awe the cute things he does! it is something new every day! I love it, I don't think he could be any cutier.