The social media hashtag - a handy way of finding all the items related to the same subject - is celebrating its 10th birthday this week.

The idea of using a # on social media is credited to an American social media expert and product designer by the name of Chris Messina, who first proposed it in a tweet made on 23rd August, 2007: "how do you feel about using # (pound) for groups. As in  [msg]?"

Happybirthday

The idea had apparently come about after he had attended a conference and friends had been annoyed at him filling their Twitter feeds with unwanted tweets about the conference. That got Messina and some others thinking about how to create a signal that would group tweets according to subject and so was born the idea of the hashtag.

The Twitter community reportedly didn't jump on board straight away - it was only after people began using the #SanDiegoFire later that year to update people on a series of forest fires in San Diego that the practice became widespread.

Twitter says an average of 125 million hashtags are now shared every day (and it's worth noting that while Twitter was the first social media to have hashtags, they have since migrated to a range of other social media platforms including Facebook and Instagram).

As well as being used to help people share information on a particular topic (particularly useful for following an unfolding event such as the recent #SolarEclipse2017 or even a TV show like #TheVoice), hashtags are also commonly used to express solidarity with a cause, such as the famous #BringBackOurGirls hashtag which appeared in relation to the kidnapping of the Chibok schoolgirls or the global #BlackLivesMatter hashtag. And then, of course, there was the #IceBucketChallenge.

The symbol used to express the hashtag, meanwhile, is known variously known as the number sign, the hash, or the pound sign. It has been around since at least the 1960s when it first appeared on telephone keypads.