Wow! that is amazing, though i dont know if that can be done on joomla or not , but if yes m ready to even pay for that though i have read an article of making a similar site of questions and answers like yahoo answers on Drupal.
I was recently searching for the same thing for my new site Vegas Answers. About the only joomla-based Q&A script I was able to find was IKNanswers. It costs $50. I'm not too familiar with Joomla so for my vegas questions and answer website, I went with a different independent script called QuestionAnswerScript that set me back $79. If you're looking to do exactly what the startup site you mentioned is doing, I'm pretty sure they are using the StackOverflow software. I believe it's free right now because it's in beta and it might be a good option for you? Good luck!
There are two commercial extensions for this Q&A. Here are their demos, SIMAnswer demo and JAnswer demo. Your choice.