Twitter has launched a new feature that lets users post tweets that disappear after a day. The social media company calls the new tweets “fleets.” The word fleet is an adjective meaning very fast or quick. Fleets can be sent as written messages, pictures or videos. Twitter says it decided to add the feature after some users expressed concerns about the current tweeting system. “Some of you tell us that tweeting is uncomfortable because it feels so public, so permanent,” the company said Tuesday in an announcement on its website. Some users also feel a lot of pressure to receive large numbers of likes and retweets, it added. Fleets cannot be retweeted and will not have likes.