Why can I remember something one day and then a different day not be able to? Like monday I study for a test and can fully understand everything I study, then the test comes tuesday and I cant reca…

From what I *recall* from some psychology classes I took: Memory has 3 stages: encoding, storage, and retrieval. Encoding is when you learn something and commit it to memory. Storage is obviously keeping all of those *somethings* inside your memory, and retrieval is pulling them out to use them (like on a test.) Failure can occur at any stage of the process. An encoding failure would mean that your brain was never even able to record something, like if you weren't paying attention in class. A storage failure would be memory decaying over time – obviously, we forget some details about past events. A retrieval failure is when you aren't able to recall something you know, even though it's stored in your memory. So, when you forget something on an exam, even though you're sure that you know it, that's a retrieval failure. Retrieval failures often produce the "it's on the tip-of-my-tongue" feeling.