They're the two best friends that anyone could have...
The prompt of the game is for both pals to give the same answer to a question about one of the friends, much like The Newlywed Game. Answer the questions correctly and you are deemed best friends for life. However, it wasn't so simple for the celebrity duo, who couldn't manage to get the questions right—except for one.
First, Fallon was tasked with determining Ruffalo's favorite sandwich. While they both came up with three-letter options, the host's "BLT" answer was no match for the Oscar nominee's preferred choice of peanut butter and jelly.
Next, Ruffalo had to figure out which physical asset on Fallon is his favorite. While the comedian modestly answered "too many," Mark managed to decide on two—his butt and eyes. Guess again, Ruffalo.
"I feel like I'm giving a Christmas gift to my wife," he joked. 'It's just so hard to get it right. There's so much pressure."