Billie Eilish said she was diagnosed with Tourette syndrome at age 11. Here are a few signs of the tic disorder.

• Billie Eilish has Tourette's syndrome, a brain disorder characterized by repetitive and involuntary sounds and motions.
• The singer spoke about her tics in a recent interview with David Letterman.

Billie Eilish has been open about having Tourette syndrome since she shared her diagnosis on Instagram in 2018.

The 20-year-old singer spoke about her experience with the tic disorder in a recent interview with David Letterman for his Netflix series, "My Next Guest Needs No Introduction."

In the interview, Eilish appeared to look away from Letterman when he asked her a question. She explained that she was "ticcing" due to her Tourette's, which causes repetitive and involuntary movements or sounds.

"The most common way that people react is they laugh because they think I'm trying to be funny. And so they go, 'Ha,'" she said, pointing. "I'm always left incredibly offended by that."

Other times, like in the interview with Letterman, people assume she's looking away from them on purpose. The host said he thought, "oh, she's fed up with me," before Eilish explained her condition.

Although she's hardly discussed the disorder in interviews, Eilish said she's happy to talk about it. Since going public with her diagnosis, she said a couple other artists shared with her that they, too, have struggled with Tourette's for years.

"I actually really like answering questions about it because it's very, very interesting, and I am incredibly confused by it," the singer told Letterman.

The singer's tics include wiggling her ears and clicking her jaw

Tourette syndrome, also called Tourette's disorder, is characterized by repetitive and unwanted motions and sounds. Most people with Tourette's experience their first tics in childhood and learn to manage their symptoms by their late teens to early 20s, according to the National Institute of Neurological Disorders and Stroke.

Eilish said she was diagnosed with Tourette syndrome at age 11. As a kid, she used to repeatedly squeeze her eyes shut or open her mouth wide, she said, demonstrating the tics.

While the tics she experiences today may not be noticeable to the average viewer, she told Letterman, "If you film me for long enough, you're going to see a lot of tics."

The singer said her "constant" tics include wiggling her ears back and forth, raising her eyebrows, clicking her jaw, and flexing her arms.

"These are things you would never notice if you're having a conversation with me, but for me, they're very exhausting," Eilish said in the interview.

She told Letterman that her tics subside when she's focused on something else or moving around, like when she's performing or riding a horse.

Other people with Tourette's may experience more complex or vocal tics

The motions that Eilish described and demonstrated are simple motor tics, meaning they involve just a few muscle groups.

Complex tics require several muscle groups to work together in distinct patterns of movement, such as hopping, twirling, or jumping, according to the Tourette Association of America.

To be diagnosed with Tourette syndrome, an individual must experience at least two simple or complex motor tics and at least one vocal tic before the age of 18, according to the association. The tics may not necessarily occur in the same time period, but they must persist for more than one year.

Vocal tics can also be simple or complex. Simple vocal tics include sniffing, throat clearing, grunting, shouting, and barking. Some people experience complex vocal tics where they blurt out words or phrases out of context.

The involuntary outbursts associated with Tourette's aren't always inappropriate or vulgar, contrary to how the disorder is portrayed in the media. About 10 to 15% of people with Tourette's experience coprolalia, a type of vocal tic that includes swear words and ethnic slurs, according to the Tourette Association.