You have to flip a cup an odd number of times to turn it over.  To make all the $9$ cups face down, you have to make an odd number of flips.  Each time you flip exactly $2$ cups, the total number of flips is an even number of times.  Hence it is impossible. 
There are $9$ water glasses, all facing up. You need to turn exactly $2$ glasses over in each time. Is it possible to turn all $9$ of them upside down after several moves? 
Options:
[a] Yes, it is possible. 
[b] No, it is impossible. 
[c] I have no idea. 