How to end an argument

Me:  Thomas, how old are you?
Thomas:  Three!
Me:  No, Henry's three.  How old are you?
Thomas:  I three (holds his little fingers up for emphasis)!
Me:  No you're not--try again.  How old are you?
Thomas:  I two!
Me:  Nope.  You're not two...
Thomas:  I big (as he walks away)!