August 13, 2013 | Celeb Watch

Justin Bieber serenaded his grandma ... naked

What is Justin Bieber doing with nothing but a guitar to cover his man bits? He's serenading his grandmother, of course.

TMZ posted the pictures yesterday, and explained that this was all part of a last minute Bieber prank that took place sometime last year.

He grabbed a guitar, headed down to his grandmother's room, and started singing, "I Loooove you grandmaaaa ... how are youuuu ... helloooo grandma."

Apparently, everyone in the house - even grandma - thought it was funny.

He could raise a lot of money for charity with that guitar. Even more if it's never been cleaned!