MY thank you!

Yesterday as we were getting into the car, I carried Evangelina into her seat, after which she very kindly said a sweet "Thank you!"  Jadon, buckling into his own seat next to her, thought it'd be funny to mimic her with a "Thank you!" as well.  Evangelina, who has quickly learned to be vigilant to protect her own possessions & rights, became upset at her brother, & with her curt voice retorted, "NO!  MY thank you!  MY thank you!"