What actually happens in the cups that perform the best? The worst? Check out some of these students' ideas:
Without a doubt, we're seeing something moving and it looks like the hot/cold/air/water (or whatever else) is doing something when it hits both the walls of the cup and the lid.
SO why not test what happens with the lid as the focus!?!?!
SO why not test what happens with the lid as the focus!?!?!