"How I Met Your Mother" star Neil Patrick Harris is recording the voiceover for Listerine's fun new educational short, "The Mouth Effect," and he's spilling the beans on his highly anticipated appearance on "Glee"! Will he sing?

"Maybe," he replies, deadpan. "There might be a tune or three."

"It's so much more labor intensive when you're filming it than when you're watching it," he says of "Glee." "When you're watching it, it's great -- there's all these fast edits and everyone's singing and dancing and it's super great fun. But when you're filming … it's a lot of work, so my hat's off to that young cast, because they're supremely talented."

As for "The Mouth Effect," Neil says he decided to lend his voice to the eight-minute film for a number of reasons: "I have a niece and nephew, and so I like being able to do darker, kind of 'Harold & Kumar,' Barney Stinson kind of roles that are a little edgier, but I also like to maintain a presence in the demographic within sort of a more educational slant."

He adds, "I grew up going to EPCOT Center and things, so I enjoy the 'narration' voice."

Blending pop culture with science and a good dose of humor, "The Mouth Effect" explores the stickiest, most populated place on earth -- the mouth -- to show why brushing and flossing alone are not enough to kill bacteria that can lead to broader health problems.

"[The film is] to sort of educate kids, adults, whoever, about not only having nice breath -- Listerine style -- but also [about] how bacteria is involved in your mouth and how to keep your teeth and gums healthy."

Watch ET for more with Neil Patrick Harris!

Related stories: 'Glee' Sings High Praises of Guest Stars  SAG's TV Winners Do Arm Curls with Trophy