I'm gonna go out on a limb and assume she already knew the answer to that question when she asked. Also agree with another poster, that she probably just wanted to embarrass you a little at that point in time in front of other family members. Lastly, dude, idk how old you are, but tickling grandma was no longer an interest of mine once I hit 10 or 11 years old. To each their own but your post makes me think you're either super young or super nervous around other girls your age, both understandable.