So the whole time the interpreter is processing this recursive function call it kinda has to remember all of the previous, how it got there because once it eventually gets down to the base cases, once it gets down to the n = 1 or 0 it actually gets a numeric response it then has to build up to it's previous reponse so fibbonacci(2) right over here is 1 + 0 fibbonacci of(2) is going to simplify to 1