'Prankster' George Clooney fills Matt Damon's backpack with rocks

George Clooney tricked his 'Monuments Men' co-star Matt Damon to carry a backpack that was filled with rocks for a gruelling hike scene.

A source said that the 52-year-old Hollywood heartthrob got his idea for pranking Damon as he was very serious about working out every morning before shooting started, Contactmusic reported.

Bill Murray and John Goodman kept teasing him while Damon exercised but he believed that he would be laughing when they didn't have stamina for the tough hiking scenes ahead.

The insider said that is all Clooney had to hear and he had the prop guys fill Matt's with solid metal bricks, which weighed about 50 pounds.

The source asserted that the other people's packs had lightweight padding but Clooney told them to act as if they felt super heavy.

The insider said that by the end of the day Damon was sweating profusely and after the scenes were canned, he dropped his pack, opened it and everyone burst out laughing when the metal bricks tumbled out.