Watch Justin Timberlake, Jimmy Fallon Sing 4 Non Blondes, Gloria Estefan

Billy Crystal, Keegan-Michael Key also appear in 'Tonight Show''s recurring "Camp Winnipesaukee" bit

Justin Timberlake, Jimmy Fallon and Billy Crystal sing pop hits in goofy 'Tonight Show' sketch "Camp Winnipesaukee."

In the latest installment of The Tonight Show's recurring "Camp Winnipesaukee" sketch, nerdy camp friends Jimmy Fallon and Justin Timberlake sing 4 Non Blonde's "What's Up" and Gloria Estefan's "Rhythm Is Gonna Get You." Billy Crystal and Keegan-Michael Key make cameos in the musical cabin, too. 

The scene opens with Timberlake and a braces-clad Fallon in their respective beds after "lights out," struggling to fall asleep. "Do you wanna sing that song we heard on the radio today?" asks the Tonight Show host, prompting the duo to belt "What's Up" while mimicking Linda Perry's distinctive, octave-leaping delivery.

Irritated camp counselor Mr. Fletcher (Key) bursts in and disrupts their late night party – kicking his legs wildly, demanding they "stop the ruckus" and threatening to revoke their "snack shack privileges." 

After they burst into song again seconds later, Mr. Fletcher storms back in the door, demanding silence inches from their faces and admitting, "I 'accidentally' rubbed poison ivy all over my nutsack." Billy Crystal arrives soon after, crooning "Rhythm Is Gonna Get You," leading to a group dance party with the Roots assisting off-screen.