Who says? Maybe her shorts are MEANT to hang around her knees like that - makes more ticklish spots more easily accessible... 😛 Plus didn't you mean Rosie, not Tiffany, Ness? :S
Yes, I agree with the double-blind experimentation. Being the son of two civil servants, I suggest completing this test in triplicate! 😀
Oh, and I think Tiffany is more ticklish. She certainly looks like she's enjoying it more. 😎