I think what Chadokoro_K is referring to is the difficulty of reviewing in Stackz. I feel like Stackz is an excellent tool for cramming large amount of words into my head on a daily basis (10s to 100s of words). However, for keeping large amounts of words in my brain, I feel like Stackz is at a bit of a disadvantage compared to programs that use spaced repetition. It would be cool to have a spaced repition mode along with the other modes if it was well implemented.