Put 6 in each, split the heavier side in two times 3 coins and set aside the other 6. See which of the set of 3 coins is heaviest, set aside the others. Now put two of the three coins on one side each. Is one heavier, that's your fake one. Are they both equal, then the one you didn't put in is the fake one.
Fun puzzle :D
Nice! Glad you had fun :)
There's a harder version :)
https://steemit.com/puzzle/@nafion/puzzle-can-you-figure-out-which-steem-coin-is-fake-part-2-hard
Oh gosh, harder indeed! I don't know how to solve that one :o