One is four, two is six
Three is one, four is one
Five is nine, six is five
Seven is nine, eight is nine
Nine is two, ten is five
Eleven is seven, twelve is eight.
What is thirteen?
Note: the format of this puzzle (two pairs per line) has nothing to do with the answer. Also, the numbers and their pairs follow the same rule, but are independent of each other. For example, I could accurately put "forty-one is five" in the list and it would not change any other pairs or their placement or anything.
I made sure to put enough pairs that an incorrect rule will not work or will be extremely far-fetched - hence the title.
This puzzle should be easy. How easy? Well, as easy as (wait for it..) pi!