We fact-checked 9 boob myths. Here's what's true.

This post is part of Fusion’s Teen Month series, a month-long dive into the lives, loves, and language of teenagers.

One of the most trying times in a tween’s life is when girls start getting their boobs. Tweens with breasts become shepherds among sheep—women among girls—and precious, boob-related information is peddled at sleepovers and wherever girls congregate.

Most of this information is garbage. But “knowing” more about an adult thing helps a tween appear more sophisticated among her friends. The only thing cooler than having boobs at age 13 is having a feigned philosophical understanding of what it means to have boobs at age 13.

Over time, just as we outgrew our Limited Too training bras, most of us outgrew our desire to be the ultimate boob truther. But some of us never took the time to definitely disprove the boob folklore we learned as tweens—and even as adult women who wear blouses and own fancy bras, we wonder if that thing Popular Sarah said in 6th grade about doing a ton of pushups is true.

To put the rumors to rest, I consulted a few experts to help clarify the most infamous boob theories I heard in middle school that still plague me as a modern adult woman.

1. Is boob size genetic? Am I predestined to look like my mother?!

Probably! But not definitely. In 2012, a team of scientists located the ~boob gene~ and found some markers that attribute breast size and density to genealogy. As Amber Guth, a breast surgeon and oncologist with NYU Medical Center, told me, boob size is kind of like any other physical trait that gets passed down the bloodline.

“Breast size, like other physical characteristics, can be genetic,” she said, “but not necessarily a direct inheritance from your mother.”

Guth said it’s also not necessarily a direct inheritance from your paternal grandmother, if you’ve heard that rumor. In short, you aren’t doomed to the same fate as all your female predecessors.

2. Can I make my boobs perkier by doing a lot of pushups?

As Jen Comas, a personal trainer and fitness expert with Girls Gone Strong, told me: This is a tricky subject. There are muscles that sit beneath the breast tissue, but Comas said they have “very little to do with the way the breasts sit and appear.”

“Strength training won’t affect the way the breasts sit, or their shape,” Comas explained. “It can develop the muscles underneath the breasts, which can make the chest, not the breasts, appear more developed.”

3. I sleep in a bra sometimes. Will this keep my boobs from sagging?

As Guth reminded me, the “sleeping in bra theory” got a re-up last year when Halle Berry famously revealed the secret behind her perfect breasts: She just sleeps in a bra! But Halle Berry, lingerie line and all, isn’t a breast expert. She’s a celebrity.

Guth told me there is “no evidence that [sleeping in a bra] helps or hurts,” and that it really just depends on your personal comfort level. So if you’ve been sleeping in a bra all these months in an effort to look like Halle Berry, sorry! Physics just don’t work that way.

4. I sleep in a bra sometimes. Is this going to give me breast cancer?

No. The reason this myth exists is because of some concern that the lower, outer edge of a bra might compress the normal flow of lymph fluid, which needs to move freely in order to carry white blood cells through the body, as well as flush out toxins, waste material, and in some cases, cancer cells. But Guth explained that this theory doesn’t make sense—the lymphatic flow from the breast goes toward the underarm, not the lower breast where a bra might pinch and cause a blockage.

If a bra were really pinching to the point of blocking any lymphatic flow, she said you would see swelling under the breast. In layman’s terms: You have nothing to worry about, but it’s still good practice to do a self-exam for any abnormal, possibly cancer-indicating lumps once a month.

5. Will eating a lot of carrots, having a lot of sex, or taking hormonal birth control make my boobs bigger?

So basically: Can you make your boobs bigger without surgery or an expensive push-up bra? While there are claims around the web that certain estrogen-rich foods—like soy, garlic, squash, and some nuts—enhance breast size, Guth said there’s “no scientific proof for this.”

Birth control pills may cause a number of changes in your body (if you’ve ever taken hormonal birth control, you know this to be true), but Guth said they don’t necessarily cause a change in breast size—and if they do, it’s not a permanent change.

Unfortunately, you can’t sex your way into bigger boobs, either. Guth told me there’s “no evidence that sex changes breast size either—so much for use it or lose it!”

6. What about that stuff Kylie Jenner says she uses—will that make my boobs bigger?

This one doesn’t date back to middle school for me, but it did catch my adult attention.

The Kardashians love products, and they love to share their products with their followers. Kylie Jenner recently disclosed the “beauty secret” that helps her maintain her curves—a product called Pure Leef that’s advertised as a natural way to stimulate fatty tissue in your boobs and butt for a plumper, Kylie Jenner-esque shape.

Tracy Pfeifer, a cosmetic surgeon with Manhattan Eye, Ear and Throat Hospital who specializes in breast surgery, told me there’s no scientific study to support the 18-year-old’s not-so-secret beauty product. She also pointed out that the lotions aren’t FDA approved, and can skirt FDA approval because they aren’t marketed as a medication.

7. If I lose weight, will my boobs shrink?

Like most other things breast-related, this depends on your own personal boob composition. Two main components make up a breast: fat (or “adipose tissue”) and glandular breast tissue, which is a bit more dense and contains the milk ducts.

“The proportion of breast tissue to fat varies greatly from person to person,” Pfeifer told me. Some women have “dense” breasts, which is a way of saying they have more breast tissue than fat. Other women have more fat than breast tissue—and fat tissue in your breasts is subject to weight loss and gains just like fat tissue anywhere else in your body.

So if you’re a woman with “dense” breasts, a change in weight likely won’t result in a change in cup size. That breast tissue isn’t going anywhere without surgery. But if you have a bit more fat tissue, it’s not unlikely to see a big change in cup size with any loss or gain in weight.

8. Is the pencil test a real thing?

Ahh, the pencil test. Coined in the `70s by fictional advice columnist Ann Landers, the pencil test is supposed to be an indicator of whether or not a woman should wear a bra. If a pencil placed beneath the bottom fold of your breast doesn’t fall (like your boob can physically grasp a pencil), you’re supposed to wear a bra.

Go ahead, try it, but this test is pretty dumb. Going braless in 2015 is very different from going braless in 1972. Times have changed! Watching a pencil fall to the ground isn’t a reason to ditch your bra if you’ve been wearing one all these years, and finding that the pencil stays put isn’t a reason to rush to the store if you’re comfortably braless. Do what makes you feel good. Forget the pencil.

9. My boobs/nipples are shaped like [insert shape here]. Is that normal?

Breasts come in all sizes and shapes—like special little snowflakes we get to carry around for most of our lives. As long as your own boobs aren’t causing much discomfort or abnormal pain and don’t undergo any strange, drastic changes in appearance—they’re healthy. And healthy is good. So stop worrying so much. Your boobs look great!

Hannah Smothers is a reporter for Fusion’s Sex & Life section, a Texpat, and a former homecoming princess.