Kudows thinking on your feet is a trained ability, one that took me a long time to acquire. Keep opening sets and when you feel like you have run out of things to stay, DONT LEAVE immediately. Stay, this will put you into an almost anxious state and will force you to think on your feet. Maybe you will get it the first try or the fifth, but just keep doing that until you feel comfortable in every situation. Oh when you are feeling anxious, take a deep breath into the pit of your stomach breath out and relax...then say something like..."What do you think about sour watermelons?" or something ridiculous to kill the tension.