It's expected that the new feature will allow users to post text, photos, videos and reactions to tweets with the tweet attached, with plans add stickers and live broadcasting.
The new ephemeral tweets bear a striking resemblance to disappearing Snapchat or Instagram Stories, and work similarly.
Twitter's goal is to serve the public conversation - it's where you go to see what's happening and talk about it.
To share a Tweet in a Fleet, tap the "Share" icon at the bottom of the Tweet and then tap, "Share in Fleet". For MacRumors readers, make sure you're following the MacRumors Twitter account where we share news stories, interesting forum threads, fun facts and tidbits, and in the future, more casual fleets.
"Fleets are for sharing momentary thoughts - they help start conversations and only stick around for 24 hours".
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In a statement, a Twitter spokesperson said the company is "always listening to feedback" and that "keeping people safe on Twitter is a top priority".
The feature was previously tested in Brazil, Italy, India, and South Korea, and Twitter said these users were more comfortable joining conversations by using the Fleets feature.
As announced via the company's blog earlier this morning, Twitter says Fleets were created to give users a way to get around the permanence and public-nature of a tweet. Fleets are only visible to anyone if they can view your full profile, and if you have open Direct Messages anyone can reply to them.
You can also respond to others' Fleets by tapping on one and sending a direct message or emoji to the creator, starting a DM message.
Similar to Instagram but unlike Snapchat, there's no current indicator to let you know if someone screenshots your "Fleet".