Does your memory amuse you?

I was thinking about this while traveling in the train across the city. I travel everyday in a reasonably fast underground train  lasting approximately 45 minutes. Since its underground I can't look out and hence I get bored at times. Also its crowded most of the times at various places. It takes 21 stops for my train to reach the destination and today I just realized that I remember what stop comes after what in the first order and the reverse order :-) yes just tested myself for 21 stops just like how you test kids to say January to December and the reverse order and everybody seems to be thrilled when the reverse order comes right... so was I :-)

Why is it that our memory is very clear and sharp at times and why is it so bad at times. Why do we forget our critical passwords at times... there could be so many questions in our minds... we sometimes forget names of people, places, etc but we remember things that are not very important.....we forget important tasks and feel stupid when reminded... we forget to report at a given place and time but remember something very trivial..... the analysis of memory amuses me...  what about you :-)