On Tuesday, Facebook CEO Mark Zuckerberg revealed the social media site will introduce a much awaited "dislike" button. Zuckerberg made the announcement during a Q & A session streamed online at Facebook headquarters.

But it won’t be simple. Zuckerberg said Facebook wants to limit use of the button to expressing sympathy when someone posts something sad or upsetting.

Zuckerberg said it was a surprisingly complicated task to build the button. But the company has an idea that it will soon begin testing.

A dislike button has been in demand for many years, but "but the company didn’t want to just give users an easy way to criticize a Facebook status."

Facebook found that most people don't want to use a "dislike" option to criticize a user's Facebook status, but as a way to express support for statuses that express something sad, like a death in the family or bad news about world events.

“What they really want is an ability to express empathy," Zuckerberg. “If you’re expressing something sad… it may not feel comfortable to ‘like’ that post, but your friends and people want to be able to express that they understand."