The health and body myths you believed when you were a kid probably seem silly to you now, like that swallowed gum stays in your stomach for seven years, or that if you make a weird face too often, it could stick that way. Weirdly, though, myths about bras are somehow different—plenty of grown women still believe them. Whether you heard them from your mom, your best friend, or an ill-informed bra salesperson, these nine common bra myths just aren't true.

Myth: Sleeping in a bra will keep your boobs perky

A couple years ago, Halle Berry revealed in an interview that she'd been sleeping in a bra since age 16. And although Berry boasts an enviable decolletage, it has nothing to do with her bra habits.

“There’s no scientific evidence that wearing a bra or not wearing a bra will make a difference with what happens to your breasts over time,” said Dr. Amber A. Guth, breast surgeon and an associate professor of surgery at NYU Langone Medical Center. “A lot of drooping and dropping happens mainly because of pregnancy, childbirth, and breast feeding.”

Then there's this little thing you may have heard of called gravity. Guth explained that over time, perkiness fades as gravity pulls the ligaments in your breasts downward. “It's just a natural process,” she said.

Myth: Wearing a bra will make your boobs sag

You may have also heard that wearing a bra makes your boobs loose their oomph. In fact, French researchers published a study in 2013 that claimed brassieres might cause women’s breasts to sag more over time because they prevent supportive muscle tissue from forming. But Guth points out your breasts don't actually contain muscle, just skin, fat, and ligaments.

“It’s not like your abs or upper arms that you can improve with working out,” she said. “Wearing a bra or not wearing a bra doesn’t change the fact that these are all physiological changes that happen to your breasts over time.”

That said, while you may not be able to boost muscles in your boobs, you can try these four exercises that strengthen your back to give your girls a flattering lift.

RELATED: 5 Myths and Facts About Sagging Breasts

Myth: Bras can cause breast cancer

This myth goes back to 1995, when a husband and wife team of medical anthropologists claimed wearing tight bras restricts lymph nodes around the breasts, thereby trapping toxins that cause cancer in the body. According to the American Cancer Society, that study failed to follow standard practices of epidemiological research, nor did it take into consideration known risk factors for breast cancer and other variables. But the myth persists on the internet, and Gwyneth Paltrow's goop newsletter even resurrected the long-debunked claim in October 2015.

Guth explains why this oft-repeated theory is bogus: “The way the lymphatics work is they go from the edge of the breasts to the nipple area, then out into the underarm. So it’s not being compressed or stopped by a bra because it’s happening away from where the underwire is.” Other studies have found there’s no significant evidence that wearing a bra is connected to breast cancer risk.

RELATED: Keep Your Breasts Healthy: How to Buy a Better (More Supportive!) Bra

Myth: You shouldn’t wash your bra too frequently

Think about it: would you sport the same pair of undies multiple times without washing them? Yeah, we didn’t think so. The same rules apply for bras, according to Jené Luciana, author of “The Bra Book.”

“The fabric breaks down when it’s constantly rubbing against the dirt an oils from our skin,” she said. “So to prolong your bra’s life, you really want to make sure you wash it after every wear.”

We know what you’re thinking: Who the heck has time to wash bras that often?! To save you some time (and sanity), Luciana recommends having a few in rotation.

“Even if it means buying three of the same bra you love,” she said. “They’ll last you longer and you won’t be wearing the same bra every day.”

Myth: Do NOT put your bra in the washing machine

This myth does hold a bit of truth. Luciana explains that bras wear out faster when tossed in the washer; straps and hooks can get caught on other items in the drum, or on the center agitator.

“It’s recommended to hand wash your bra in cold water with a gentle detergent or baby shampoo,” she said.

But as long as you accept that you'll have to replace your lingerie more frequently, there's nothing wrong with washing your bras in the machine.

“I’m a realist, and a busy mom, so who the heck has time for that?” Luciana admitted.

Instead, she advises using a cool water cycle, a gentle detergent (look for "free and clear" on the label), and placing your bras in a mesh garment bag.

RELATED: The Best Laundry Detergents for Sensitive Skin

Myth: You can keep your bra forever

If the bra still fits, wear it, right? Not so much. Even if your boobs stay the same size, you shouldn't hang on to your favorite bra for all eternity.

“A general rule of thumb is a year,” Luciana said. “But if it’s a bra you don’t wear very often, you could have it for three years. It all depends on how often you wear it and how well you take care of it.” Toss bras when they're fraying around the edges, the cups are stretched out, the underwire is bent, the tops are falling down, or you've started using the tightest row of hooks.

Myth: Your bra size is the same brand-by-brand

Just because the bra fitters at Victoria’s Secret determined you’re a 34C doesn’t mean that’s your size for every brand, or even every style.  All of these brands use a “fit model,” Luciana explained. “There isn’t a machine that determines the size of a bra. They actually look at a real woman, determine her size, and then base the rest of their line on that person.” Since there’s absolutely no consistency, “you might have five different bras in five different sizes from five different companies,” Luciana said. That means you really need to try them on at every store to find the right fit.

Myth: A new bra should fit at the last hook

Luciana said buying a bra to fit perfectly at the largest band size isn’t necessarily the best method. Instead, she recommends fitting a bra to the middle hook. “That way you can move to the outside hook if you’re on your period, since women tend to get a little bigger during that time. And if it gets a little stretched out, then you move it to the tightest setting.”

RELATED: What the Perfect Breast Looks Like, According to Men and Women

Myth: There’s one way to put on your bra

Step 1: Place bra upside down and inside out. Step 2: Hook bra in front of torso. Step 3: Twist bra around and pull straps onto shoulders.

Those three steps describe the most common way to put on a bra, but Luciana has a totally different method to help your breasts sit comfortably in their cups. “The idea is that you’re leaning forward, hooking it in the back, and fixing the straps over your shoulders. Then you stick your hands into the cups, and scoop your breasts into the center,” she explains.

This article originally appeared on Health.com.