What's wrong with my study routine that always leaves me forgetful?

When you're in the library for hours on end, it's easy to get caught up in the idea that you're learning, but if you can't recall anything the next day, there's a problem.

I used to be that kid in college who thought studying meant just re-reading my notes for hours. I'd show up to classes feeling prepared but walk out with a brain full of blank spaces. It was like I was drinking from a firehose and most of it never stuck. It took me years to figure out why - not because I wasn't smart, but because it's actually really hard to remember something when you're just re-reading it over and over again.

What's your secret to learning retention?