Memory Lapse

I just spent the last half hour or so chatting with friends and trying to remember a phone number. I could remember the last 4 digits of this number, but not the first 3, and I couldnt' remember whose number it was. I went over my list of friends but couldn't find one who matched that number. I was beginning to go crazy because I knew that these 4 digits were not something my mind randomly created. Then I remembered. They are the last 4 digits of my social security number. It is 2:30am and I just finished my homework for statics and differential equations.