The boys’ brains have matured enough that they can now have 100% verbal arguments – about nothing.  For example, both boys are in their respective car seats and it begins:

Benjamin:    Taxi cab!

Jonah:   No taxicab!

Benjamin:    Yes taxi cab!

Jonah:   No taxicab!

Benjamin:    TAXI CAB!  TAXI CAB!

Jonah:   NO!  NO TAXI CAB!  NO!

.... quickly disintegrating into simultaneous exclamations of mental anguish and general malaise.