Pictures of Bieber in the act, have been obtained. He is seen standing only with his guitar covering his privates. The pictures are from October 2012, reports tmz.com.
According to sources, Bieber had slept for a little too late on Thanksgiving morning and woke up after hearing family and friends stirring around the house.
He decided to prank them by grabbing his guitar and playing a naked set a la Jenny from "Forrest Gump".
He went up to his grandmother and started singing impromptu lyrics like, "I Love you grandma ... how are you ... hello grandma."
Beiber's grandmother found it funny, and everyone in the room started laughing too. But, the singer was later asked to go back to his room and wear some clothes.