Say What?

Caleb was chattering on the other day, saying "na na!" over and over. I tried to get him to say "mama" instead. This is how the conversation went:

Me: Caleb, say 'mama!'
Caleb: Na na!
Me: No, mmma-mmma!
Caleb: Na na!
Me: Mama!
Caleb: Na na!
Me: Mama!
Caleb: Dada!