Actual dialogue with Vivi this morning:
Vivi: "Mama."
Me: "You said, 'Mama,' Vivi! Good job! Say, "Mama.'"
Vivi (more quietly, but grinning): "Mama."
Me: "Good job! Can you say, 'Dada'?"
Vivi (louder, and grinning wider): "Mama."
Me: "No! You tricked me! Say, 'Dada' for me."
Vivi (looking right into my eyes): "Mama."
Only 14 months old, and already contrarian!