For me this always works (can’t overdo it with the same girl though). Point out something random around you, and completely make something up about it. For example, point to a nearby painting or sculpture or whatever, and say, “did you know that that painting was created in 1852 in honor of Abraham Lincoln’s birthday?!” When she says, “really?”, or “how do you know?”, you say, “oh, I have no idea – I made it up!” Gets a laugh every time.