Memory

On Thursday at the audition, I saw a girl that I used to play with in 1st grade. We weren't in the same class, but she was really nice and I played with her. So I went up and said hi. She didn't remember me. I thought she would, so I was kind of disappointed. I wonder if she remembered me a bit after the audition or if she still thinks I am a complete stranger. Why does memory work like it does? I can remember that girl from 7 years ago AND my lines from a play 3 years ago but I can't remember the commutative property of whatever that I studied in algebra a month ago. My great-grandma can remember stuff that happened to her in the '30s but can't remember what she did yesterday. The mind does not make much sense.