Goku and Uub were training in the Hyperbolic Time Chamber. One day Goku said, "Uub, I want to see how far you've come after facing all those villains like Omega Shenron and Baby. I want to give you the ultimate test: Try to kill me." Uub was suprised. "W-What? You want me to try to KILL you?!?" To this, Goku said, "Yes. And I will try to kill you." Before Uub could say anything else, Goku turned into a Super Saiyan and punched Uub to send him flying. Uub shouted, "I won't let you down, Goku!!" and shot a Super Kamehameha at Goku(Goku had taught Uub this attack). Goku dodges it and fires a Kamehameha of his own. Uub gets hit by the attack, but is easily able to get back up and fight again. He charges at Goku and punches him in the face.He then smashes him into the ground, which Goku uses to his advantage as he grabs Uub by the neck and throws him into the air. Uub recovers just quickly enough to see a 10x Kamehameha heading right toward him. He knows he can't do anything about it, so he braces himself for impact.
But it never hits him.
He turns around and sees it fly off into space. He turns to Goku and says, "Why didn't you have it hit me?"
Goku just says, "Uub, I'm glad you realized that you can't handle anything. But that's not the reason it missed you."
Uub says, "Huh?"
(Goku transforms into a Super Saiyan 4)
"That just wouldn't be any fun if you died so soon."