[omk3]

Originally Posted by Catire

or

Spoiler:

Split the coins into 3 groups of 4 coins (A,B and C)

First you weigh A and B, if they have the same weight then the coin is in group C, if either of them weight more or less than the other then the coin is in that group. Discard the lighter or heavier coin groups.

You then split the group where the coin is into two groups of two and weigh them, wichever is heavier or lighter thats where the coin is. Discard the heavier or lighter group.

Then you just weigh the two coins left and you've found the fake one.

But:

Spoiler:

First, you used the scales more than two times.
Second, you say

and weigh them, wichever is heavier or lighter thats where the coin is.

Obviously one is heavier, one is lighter. Which one has the fake coin? You don't know if the fake coin is lighter or heavier.

(Edit) Sorry, I answered before you deleted your solution...