Why is there a crack in my ceiling?

As I was falling asleep last night I noticed a loooooong crack in my ceiling, and wondered to myself "how long has that been up there?"... I must have fallen asleep under that crack 760 times (365 days a year x 2 + 20 days prorated - 10 days away on vacation) That is a long time to stare at something and not even know it was there.... kinda like the 2nd Kardashian sister... ha... but I digress. I really wonder how long that crack has been there and how it got there.




See, it's not a particularly WIDE crack... just long and skinny. Makes me wonder if there used to be a wall here.