Britney Spears may have been billed as the musical guest on Tuesday night's "Jimmy Kimmel Live," but the pop star also used the appearance as a chance to show off her funny side.

In one bit, Spears was visited by Kimmel in her dressing room. After some casual hellos, Spears asked the talk-show host, "Oh, did you get it?"

Kimmel said, "I did get it."

Spears excitedly responded, "Oh my gosh, let me see. Really? Great! Oh, you got it."

Then came the big reveal: Kimmel pulled up his pants legs to reveal that he had gotten the words "Britney Spears" inked on his calves. Spears, shocked by his dedication to their oath to get matching tattoos, said, "Oh my god!" Then she showed off her own: Written in tiny letters on her wrist were the words "Jimmy Kimmel."

"It's in pen," he said, disappointed by the fact that hers is not permanent.

"It's ink," she responded. "Shhh. You're getting on my nerves," she added as she escorted him out.

Later in the show, Johnny Knoxville revealed that Spears had originally been in "Jackass 3D," and that now the footage was finally being shown. In a pre-taped bit, Spears arrived on the set, telling Knoxville and his crew of merry men, "I'm ready! I'm not scared of poo-poo. Let's do it!"

She looked into the camera, before stepping into the Porta-Potty, saying, "Hi, I'm Britney Spears and this is the poo cocktail supreme."

Then a blonde got swished around in the Porta-Potty, dirty water splashing everywhere. After the stunt, the blonde, whose face you couldn't see, got out and began chasing the guys around.

After she taped her stunt, Spears took to Twitter to send a special message to Knoxville. "Just did my first @jackassworld stunt with Knoxville and company. Knoxville better watch his back," she wrote.

The pop star came out one more time, this time for her musical spot. While much of the performance was similar to recent gigs in Las Vegas and San Francisco, this time Spears, starting with "Till the World Ends," wore a brand-new look. Her sparkly black and green bodysuit was paired with black fingerless gloves as she sashayed her way around the stage, much to the delight of the crowd.

For her second song, "Hold It Against Me," she rocked bright red sparkly fingerless gloves with a super-tight, sexy black bodysuit with sparkles.

Spears dropped her latest album, Femme Fatale, this week, and her MTV special, "Britney Spears: I Am the Femme Fatale," airs on MTV this Sunday (April 3) at 9 p.m.