• Fallon reads out a question to test his friendship with Rufallo on his night show.

Fallon reads out a question to test his friendship with Rufallo on his night show. (Photo : YouTube / The Tonight Show Starring Jimmy Fallon)

In honor of the National Best Friends Day, which is among the Internet's unofficial holidays, Mark Rufallo appeared on Jimmy Fallon's "The Tonight Show" on June 8, Wednesday, to take a friendship test when "The Best Friends Challenge" kicked off. The duo wanted to prove that they are sharing something beautiful - friendship.

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The game would prompt both friends to give identical answers to a question about them.  The pair would then be deemed as best friends when they answer the questions correctly. The questions were quite difficult that Rufallo jested he feels like giving his wife a Christmas gift.

'It's just so hard to get it right," the "Avengers" star said. "There's so much pressure."

The first question was Rufallo's favorite sandwich. The show host answered "BLT" and Rufallo answered peanut butter and jelly.

The second question was Fallon's favorite physical asset in which Rufallo answered butt and eyes. However, Fallon's answer was "too many."

The third question was Rufallo's biggest fear. According to the award-winning actor, it is drowning in milk. However, Fallon thought it is being alone in the world.

The two friends almost lost hope, but the last question was who their favorite super hero is. They excitedly answered "Hulk" and that was the first time they came up with identical answers. Rufallo played Hulk in many "Avengers" franchise.

The duo jumped and hugged with joy for finally getting the correct answer and "The Tonight Show" host declared they are "Best friends forever!"

Rufallo stars in "Now You See Me 2" that is currently in theaters where he reprises his role from the original film released in 2013. One notable incident when the movie was filmed was when the actor threw up for real in a drowning scene.

Rufallo blamed the vegan cheesecake of co-star Woody Harrelson which upset his stomach, according to Stuff. Harrelson also reprises his role from the 2013 action thriller.

As for his Bruce Banner/Hulk character, the 2017 "Thor: Ragnarok" will have a Banner against Hulk scene, the Oscar-nominated star told Collider during a "Now You See Me 2" final press day interview.

Watch Rufallo and Fallon putting their friendship to test on the latter's night show.