Scene: Will's bathtub on Thursday night. Zach is out of town until Sunday. Will is playing and splashing happily in the tub.
Me: Will, can you say "Mama"?
Will (looking straight at me): Da Da.
Me: No, not Dada. MA-MA.
Will: Da Da.
Me: No, Will. Mama. MA. MA.
Will: DA. DA.
This continues for several more iterations, until, defeated, I give up.