A New Twist on an Old Grading Paradigm

"You all have A's at this point" is a not uncommon opening day remark in school.  I am not one of the teachers who say that because I don't think of grades this way (I think of it is as an accumulation of effort and performance as the year goes along).  However, I do like to a play a little game with this concept near the end of the first marking period.  Here's how it usually plays out:

Me: "I have been thinking of a new way to do grades"
class looks slightly intrigued
Me: "I was thinking of starting everyone off with 100's and take away points rather than adding them"
some students looking serious, some looking disturbed
Me: "As the years goes on and as you make less than perfect scores the 100 would go down"
some students (seeing where this is headed) look distraught, some still going along
Me: "And then that fateful day would come when you would get down to the 89 and know that the A was never coming back"
all students looking horrified but some now realizing it is a joke
Me: "What do you think, sound like a good idea?"