Before Twitter came along, no-one really knew what the point of the hash key was on the keyboard (except for maybe computer programmers).
The social media network transformed its use, making it a way to tag tweets and the hashtag became a thing.
Ten years on, the # symbol is used 125 million times on the platform each day.
Here are some of the biggest hashtags ever, mentioning big events, controversies and creating Twitter moments in the process.
Black Lives Matter is an international movement that first mobilised on Twitter, bringing people together to work towards creating a fairer society for black people.
BLM now has an online forum and charity, which it says is "intended to build connections between black people and our allies to fight anti-black racism, to spark dialogue among black people, and to facilitate the types of connections necessary to encourage social action and engagement".
Politicians don't have the best reputation when it comes to using Twitter, despite Ed Miliband's best efforts to try to change all that.
Since 28 April 2011, the world has celebrated former Labour MP Ed Balls' Twitter fail, when he simply tweeted his own name.
People obviously still find it funny, which is probably why it has its own anniversary every year.
Is it blue and black? Is it white and gold? Two years on, the debate still rumbles on.
It all started over this photo of a dress and the world couldn't agree on the colour of it.
It actually turned in to a huge scientific experiment about how humans see colour. But at the time it made you delete half your Facebook friends list and ignore every family member that didn't agree with you.
HeForShe is a United Nations Women's campaign for gender equality, which started in 2014 and is backed by the likes of Emma Watson and Simon Pegg.
The campaign aims to inspire men to understand and end inequalities experienced by women and show how gender equality affects people in all social, economic and political situations.
People used the hashtag as a platform to express their own battles with gender inequality in everyday life and campaign for change.
Twenty-five-year-old Australian cricketer Phillip Hughes' sudden death in 2014 shocked fans around the world and triggered tribute messages on Twitter using the hashtag #PutYourBatsOut.
It all started when fan Paul Taylor put a cricket bat outside his house as a mark of respect.
It then started trending on Twitter, with current and former cricket stars, celebrities and fans from around the world also putting out their bats as tribute.
Last year, a dad from north Devon made an urgent plea on Twitter to find a replacement sippy cup for his autistic son Ben.
The tweet - with spawned the hashtag #CupForBen - was retweeted more than 12,000 times.
The 14-year-old had only drunk from the double-handled cups, which are no longer produced, since the age of two.
It all ended in success when Northumberland-based Tommee Tippee said it would produce 500 new cups after it searched factories worldwide and found the original mould.
The Ice Bucket Challenge went viral in 2014.
It involved people pouring cold water over themselves and posting the video on social media.
ALS sufferer Pete Frates made it popular when he had freezing water poured over him but the trend itself started with this video...
It raised close to £100m for charity and funded an important scientific gene discovery in the progressive neurodegenerative disease ALS.
This one was started by comedian Sarah Millican.
She created an online community for anyone feeling lonely in 2011 and followed it up with various Twitter campaigns, especially at Christmas.
She told the Evening Standard how the idea came about: "It started because I'm a big softie and can't bear the thought of people being alone on Christmas Day."
This hashtag was the first to really take off on Twitter.
Basically, you promote people's handles who you think your followers might want to follow.
The first #FollowFriday was on 16 January 2009 and there have been more than half a billion #FF (or #followfriday) uses since then.
Not one you've heard of?
Maybe that's because it was the first hashtag ever used on Twitter and was posted by former Uber and Google engineer, Chris Messina.