A feature of StackExchange chat, "oneboxing", automatically inlines some links that are posted on a single line by themselves.

Some links will be automatically inlined if posted on a single line by themselves, such as:

  • Stack Exchange questions, answers, and users
  • Chat messages and rooms
  • Images
  • Wikipedia pages
  • Youtube videos

We call this "oneboxing". For example, entering the following in chat on a line by itself …

https://bicycles.stackexchange.com/questions/1678/what-are-the-reasons-for-and-against-wearing-a-bicycle-helmet

… will produce a onebox for that link, like so:

Sample oneboxed link

For more information on what links produce oneboxes, see Which links and sites are handled specially in chat?