And of course, they sand Trainor's breakthrough hit "All About That Bass."
During that car ride, Trainor told Corden that one person she was truly starstruck over was none other than TV's Dr. Phil.
Would you know it? At a stoplight, that's who hopped into the back seat.
Trainor had a little mini-session with the good doctor, talking about her struggles with anxiety.
Dr. Phil even joined in the sing-along.
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