Watch Jake Gyllenhaal Pull a "Salmon" Prank on David Letterman's Legs

Why is the actor's hand in between the talk show host's thighs?

The next time someone flaps their hand around in between your legs, it just might be Jake Gyllenhaal.

The Nightcrawler actor appeared as a guest on the Late Show on Wednesday, and David Letterman learned that Gyllenhaal does not keep his hands to himself.

Read more Jake Gyllenhaal, Director Dan Gilroy Explain Why 'Nightcrawler' Doesn't Have Any Sex Scenes

"That's the nicest thing anybody's done to me," Letterman joked about the prank Gyllenhaal pulled on him.

The Donnie Darko actor deemed it a "visceral experience," and Letterman seemed to agree.

Watch Gyllenhaal's behavior and Letterman's surprising reaction in the video below.

Twitter: @_RyanGajewski