Why am I so forgetful?

I spent a half hour around my house today looking for my AP Calc book and it wasn't here. I remember I had the book last night to finish up some problem sets and I took the book to school even though the next time my AP Calc cl meets in Monday. I drove to school after searching my house, checked my locker and there it was. As hard as I try, I can't remember ever putting the book inside my locker. The biggest problem though, is how sometimes i'm so forgetful I forget to write things down so I won't forget them. If let's say i'm about to write my homework ignment down and someone comes in and says "Hey Jim" or "What's up Jim?". I usually forget completely what I was doing before I was slightly interrupted and I wouldn't realize i'd forgotten something until it's too late.