New Delhi, Dec 30 : Bollywood diva Sara Ali Khan, who is famous for her 'Knock Knock' jokes on the social media, exchanged a playful banter with megastar Amitabh Bachchan on the sets of the show 'Kaun Banega Crorepati 15'. The 'Kedarnath' fame actress appeared on the finale episode of the quiz-based reality show, along with her grandmother, legendary actress Sharmila Tagore. After the host, Amitabh welcomed the duo on the hot seat, Sara said: 'Amit sir, knock knock.' The cine icon was left confused and asked what is "knock knock." Replying to him, Sara said that he has to ask 'who's there'? The 'Brahmastra' actor nodded to the gorgeous actress and said: 'Yes, who's there'? Sara started the banter saying 'Watch', to which Amitabh said: 'Watch who'? The 'Simmba' actress said laughing, 'Watch us win Rs 1 crore.' The 'Sholay' actor then asked Sara: 'Do you think you're the only one who's popular? I also know knock-knock.' 'Knock knock,' said Big B, and Sara replied: 'Who's there'? Amitabh said: 'Dense,' to which Sara said: 'Dense who'? The 81-year-old actor said: I'm impressed with your 'confidence'. I hope that you will win crores of rupees today. And the special cause for which you both will play today, it will be donated towards it.' The 'Gaslight' actress is known to play 'Knock Knock' jokes often with her brother Ibrahim Ali Khan. Sara is the daughter of actors Saif Ali Khan and Amrita Singh. Her grandparents are Mansoor Ali Khan Pataudi and Sharmila Tagore. Saif and Amrita had announced their separation in 2004. Saif tied the knot for the second time with actress Kareena Kapoor in 2012. They have two sons Taimur and Jeh. On the work front, Sara next has 'Ae Watan Mere Watan', 'Metro... In Dino', Jagan Shakti's untitled project, and 'Murder Mubarak'. /IANS