Goku and Mr. Popo in “Unfinished Business”

I was wrong, Popo. We're not the same. He's much stronger than me.

Listen to me, Goku. It is as I told you before dear boy, your speed and skill are matched evenly with the doll. It's just that you haven't finished your mind training.

Mind training...

A warrior's strength is always important in a battle, but it's not enough. The mind is also a powerful weapon and must be developed just like your muscles. If you do not master your mind training, you will never be able to defeat the doll!