what one thing did you never really understand and then one day *poof* it made sense?

for me it was the whole "top of/bottom of" the hour thing. i could never figure out what the bottom of the hour could be. i assumed it was 6:00 if it was 5:00...but that's the top of the hour. i didn't get why 5:30 was the bottom of the hour until i realized it's the beginning of the second half of the hour, or the bottom half.