So today, I tried putting a bit of Easy Cheese on her back. She circled around a few times, then sat down, thinking it would fall off. She got up and turned around, but couldn't find anything. Then she noticed it was still on her back, so she started circling around again, sitting down, getting up, circling around again, etc. etc.
Finally I took pity on her and scraped the cheese off her back with a cookie. Still feeling a bit evil, I promptly stuck the cookie and cheese to the top of her nose. Her reaction had me laughing so hard I couldn't breathe. She started snapping at thin air, as if she could see the treat in front of her and was trying to snatch it out of the air.
I must do this again some time with a camera rolling. This would make a hit video on YouTube.
(I eventually picked the cookie off her snout and gave it too her.)