Zac Efron, Robert De Niro Face Off in Shirtless Body Contest, Flex and Do Push-Ups Together — Plus, Is De Niro's Thumb Where We Think It Is?!

Robert De Niro and Zac Efron go shirtless -- and get crazy! -- on set of Dirty Grandpa Credit: FameFlynet Pictures

We’re not sure what’s happening, but we don’t hate it. Robert De Niro and Zac Efron have been photographed getting mighty close on the set of Dirty Grandpa in Tybee Island, Georgia. The 71-year-old Oscar-winning legend ditched his shirt and joined the hottie, 27, in a shirtless, muscle-flexing contest — and it only got wilder and crazier from there.

The outrageous pics give a big, surprising glimpse at their upcoming comedy, which tells the story of an uptight guy (Efron) who is tricked into driving his perverted, former Army general of a grandfather (De Niro) to Florida for spring break.

De Niro, who impressively bared his abs alongside shirtless award-winning Efron, clearly portrays the perverted grandpa in the flick. It’s safe to say the two actors, who have found themselves in bouts of laughter, are clearly having a blast working together. Somehow, the Goodfellas star even found his thumb up the High School Musical alum’s bum…!

The flick, written by Da Ali G Show writer Dan Mazer, is set to be released in 2016.

Tell Us: Does De Niro or Efron get your vote for the shirtless contest?!