Tweets, which may include hyperlinks, are limited to 140 characters, due to the constraints of Twitter's Short Message Service (SMS) delivery system. Because tweets can be delivered to followers in real time, they might seem like instant messages to the novice user. But unlike IMs that disappear when the user closes the application, tweets are also posted on the Twitter website. They are permanent, they are searchable and they are public. Anyone can search tweets on Twitter, whether they are a member or not.