While conventional gender stereotypes are deeply ingrained in many of us, more and more young people are starting to challenge them.
A recent survey revealed that half of America's millennials (people currently aged 20 to 35) felt gender isn't limited to male and female. They've even been called the "gender fluid generation".
In 2015, pop star Miley Cyrus, 23, spoke out about her gender identity, telling Out magazine, "I don't relate to what people would say defines a girl or a boy, and I think that's I had to understand. Being a girl isn't what I hate, it's the box that I get put into."
A year before, Orange Is The New Black star Ruby Rose had also said, "I am very gender fluid and feel more like I wake up every day sort of gender neutral."
Technology, it seems, is catching up, as Facebook now gives users more than 50 gender identity options, including "agender" and "gender fluid".
Rebekka Howie, from Paisley, Scotland, felt different from an early age and prefers to be referred to by the pronoun "they" rather than "he" or "she".
"I was born female but, even as a teenager, when I started to go out with friends, I realised I felt different. Sometimes I felt I could be more manly in my personality,' says Rebekka, now 17.
"I had an unusual style and never stuck to 'female' clothes – I just wore what I liked. I'd always been attracted to girls but, at my Catholic school, being gay just wasn't accepted. I had no friends, and everyone turned against me because of it.
"Eventually, I didn't want to hide any more. I didn't care what people thought, I just wanted to be me."
Sadly, Rebekka's schoolmates didn't understand, and this led to bullying. In 2011, aged 12, Rebekka was forced to leave her school and start elsewhere. It was then that Rebekka and her mum, Erica, 38, spoke about being gay.
"It was quite hard to tell Mum," Rebekka says. "She's got her own strong opinions and was brought up in a very strict family, so she wasn't very understanding at first.
"But as the years have passed by, she's finally accepted me. It was incredibly hard, but fortunately I had my nan and auntie, who helped and supported me along the way."
While Rebekka mostly wears what would be described as more "masculine" clothing, like skinny jeans and band T-shirts, some days are also spent in more traditionally "female" garments.
Rebekka says, "Mum couldn't understand why I wore men's boxers and sports bras, so I had to explain that sometimes I woke up feeling more feminine and other times more masculine.
"It was difficult for her to accept, because she wanted a daughter... a girly girl."
During her early teen years, Rebekka had just assumed she was more of a tomboy. But in 2013, aged 14, Rebekka heard the term "gender fluid" and identified with it.
"I was at Gay Pride and hanging out with transgender friends, and I overheard them talking about gender fluidity. I'd never heard of it before but it almost sounded familiar," Rebekka explains.
"I looked it up on the internet when I got home and instantly connected with it – everything suddenly made sense to me."
The teen soon learnt that those who are gender fluid may identify as male, female, non-binary or a combination of these at any time.
"Realising I was gender fluid really helped," says Rebekka. "It means my personality can be masculine or feminine. Sometimes, I'll wake up and feel more one than the other, and some days I feel gender neutral."
Over time, Rebekka's mum has learnt to accept her child as gender fluid.
"As long as I'm happy, Mum doesn't mind," Rebekka says. "But in general, people don't understand it. They think gender fluid people are transgender.
"It's not as widely talked about, so I'll often be asked why I look like a girl one day and a boy the next.
"Now, I prefer to use the pronoun 'they' as it's gender non-specific. Most of the time, I wear more sporty and typically masculine clothes. But I do sometimes wake up and want to do my brows and wear a dress – it just depends!"
Since leaving school, Rebekka has started at college and now wants to campaign to raise awareness about different gender identities.
"It's great that someone as big as Miley Cyrus is behind us. It's taken years for transgender people to become accepted, and I think it will take a while for people to understand gender fluidity," Rebekka says.
Pairing up with the charity Fixers, which encourages young people to use their past experiences to improve their future, Rebekka has made a video along with friends from the LGBTQ community, urging others to be more aware of gender fluid and transgender issues.
"I wish people would be more understanding and not tell me who or what I should be. Society places so much unnecessary importance on male and female genders.
"It shouldn't matter – I'm just me."
For more information, visit fixers.org.uk