Brooke Shields Shares Her Unusual Technique for Getting Her Iconic Eyebrows

Brooke Shields is finally sharing the secret behind her bold eyebrows.

Known for her signature bushy arches, Shields, 56, filmed herself grooming her eyebrows with the two products she swears by. And — spoiler alert — one of her go-tos isn't actual makeup at all.

"Here's how I create THE eyebrows! Thoughts, feelings, emotions?" she captioned the Instagram video.

Instead of grabbing an eyebrow pencil to fill in her arches, Shields prefers to use the Prismacolor Ebony Graphite Drawing Pencil used by artists for sketching — and unauthorized by makeup pros. "It's actually for an artist to draw," Shields said with a laugh before using the deep pencil to fill in sparse spots.

After that, the rest of her brow routine is relatively simple. Shields grabs the Trish McEvoy Brow Perfector clear pomade and gently swipes it along her arches to keep them in place.

"It keeps them really sort of pretty and fresh. There we go!" Shields said as she finished.

While Shields' Prismacolor Ebony Graphite Drawing Pencil gives her the dark brows she loves, dermatologists warn about using non-makeup pencils on the face.

"Pencil tips contain graphite, and while nowadays they are a mixture of graphite and clay, getting a poke from the end of a pencil often leaves a permanent tattoo mark behind," explains New York-based board-certified dermatologist Dr. Kavita Mariwalla.

When eyebrow hairs get tweezed, it becomes easier for the graphite to get "embedded in the skin," Dr. Mariwalla says. "It gets embedded and is not purged out on its own. If you start using graphite pencils in areas like the brow, you run the risk of a similar [permanent] mark in this hair-bearing area. This is because when plucked or shaped, you can have microscopic areas of skin breakage."

Beverly Hills-based celebrity board-certified dermatologist Dr. Harold Lancer agrees. "It can have some negative effects. Graphite may cause an allergic reaction in the skin or if the particles fall near lashes, which can also irritate the eyes. My recommendation is to use cosmetic pencils specifically designed for this use, such as powder and wax formulas," Dr. Lancer tells PEOPLE

While Dr. Mariwalla says people don't particularly experience infection by using graphite drawing pencils on the skin, she also still recommends actual makeup above all else. "When eyebrow pencils are formulated, they are done so by cosmetic chemists and tested to make sure they are safe around the eyes," she says. "While a colored pencil may work I would not recommend doing so on a regular basis as it can tattoo the skin and it may not be able to be removed."

Fans immediately had lots to say about Shields' beauty secret. "I feel like I've stumbled across the Holy Grail. Prismacolor ... brilliant. Raiding craft supplies now!" one person commented.

Someone else said, "Your willingness to be transparent for women our age is a gift 👏🔥."

While Shields' envy-inducing brows are one of her most famous assets, she has admitted that she doesn't pay them too much attention. "I, for some reason, don't really think about them," she told PEOPLE in 2017. "I think because I've never really groomed them, so it's less of an issue. I did once, and it was a disaster—I let someone get ahold of me, and I was like, 'Nope, not anymore.'"

She admitted that they "changed over the years," but now she likes to let her brows be. "It's funny because I was recently photographed, and Carine Roitfeld wanted the makeup artist to really enhance my brows so that they could look like they did in the 80s. I looked like a clown," Shields remembered.

"That sort of the natural, unruliness of it, wasn't a part of my real life anymore," she continued. "My hair quality is different. They've changed, definitely. I just try not to become too aware of them."