Today is the tenth anniversary of the first tweet containing a hashtag. Since then, the humble typographical symbol has become part of everyday life, shaping online discussions about everything from race and gender to the colour of THAT dress. According to the BBC, they appear in 125 million tweets every day.
Here are some of the hashtags that have left their mark on Twitter - and, in some cases, the world:
#YesAllWomen
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In 2014, 22-year-old Elliot Rodger authored a "manifesto" detailing his hatred of women before embarking on a deadly knife and gun rampage in Isla Vista, California, which left six people dead.
The hashtag "#YesAllWomen" sprang up in response to some male tweeters who attempted to stifle the debate about the dangers of misogyny by protesting that "not all men" are sexist.
Women shared their own experiences with sexism and sexual violence under the hashtag, as a reminder that misogyny affects all women regardless of how many men are perpetrators.
#BlackLivesMatter
Several high-profile deaths of unarmed black men at the hands of white people and police officers have sparked outrage in the African-American community in recent years, starting with the 2012 killing of teenager Trayvon Martin by a white neighbourhood watch volunteer.
The seemingly low value put on the lives of black boys and men - particularly by the police - gave rise to the hashtag "#BlackLivesMatter, which in turn ultimately inspired a grassroots protest movement.
While Black Lives Matter remains controversial - opponents have accused activists of violence, vandalism and disorder at protests - its high profile has put black communities' long-standing concerns about police brutality and institutional racism on a national platform.
#JeSuisCharlie
The hashtag #JeSuisCharlie ("I am Charlie") spread across Twitter in January 2015 after a gun attack on the office of French satirical magazine Charlie Hebdo in which 12 people were killed by two Islamist terrorists.
Coined by French journalist Joachim Roncin on the day of the attack, the pro-free speech rallying cry "was … used 1.5 million times that day and about six million times over the next week on Twitter", says the BBC.
#JeSuisCharlie introduced the concept of "hashtag solidarity". Two years on, every terror attack seems to generate its own "#PrayFor" or "WeStandWith" hashtag - although some have warned that social media condolences are no substitute for real-life engagement.
#EdBallsDay
Simple but effective, #EdBallsDay encapsulates the best of Twitter humour. On 28 April 2011, then-Labour MP Ed Balls was seemingly indulging in a guilty past-time that most of us will recognise - searching the internet for mentions of himself - when he accidentally tweeted his own name.
The rare sight of a public figure being caught in a moment of oh-so-human vanity was like blood in the water to British Twitter, with so many people posting their own "ed balls" tweets that his name continued to trend for hours.
If Balls was hoping the embarrassing incident would blow over, he was mistaken: every year, on 28 April, Twitter users continue to celebrate "Ed Balls Day" by sending a commemorative tweet containing only the politician's name.
#MomTexts
US late-night host Jimmy Fallon has mastered the art of using his TV audience to take over Twitter. Hashtags he has sent trending include "#WhyImSingle" and "#IThoughtIWasCool", but #MomTexts, in which users were encouraged to share funny, weird or cute text exchanges with their mothers, has proven the most enduring:
#AddaWordRuinaMovie
Hashtags encouraging users to put an unusual spin on film and song titles are a near-daily feature of Twitter's trending list.
With candidates like #Toryboybands ("Camaroon 5") and #explainafilmplotbadly (Home Alone: "a child victim of neglect repeatedly maims a pair of criminals"), it's hard to pick a favourite, but #AddaWordRuinaMovie certainly got creative juices flowing:
