This is my favorite brain teaser/logic puzzle, by far. I learned it in my high school math class one morning and spent the rest of the day figuring it out. It comes in two parts.

1) The Heavier Coin

You have a balance and twelve coins, one of which is heavier than the rest, but you don’t know which one it is. Using just 3 weighings on the balance, can you identify which coin is the odd one out?

Okay, you’re probably wondering why that’s so hard, right? It’s a pretty simple puzzle. (I’ll not ruin it in case you haven’t figured it out, though.)

Let’s make it harder by making one very simple change — instead of me telling you that the odd coin is heavier than the other 11, I’m going to make it simply “different.” It could be lighter. It could be heavier. Who knows? Everything else stay the same. Here’s the new question:

2) The Different Coin

You have a balance and twelve coins, one of which weighs a different amount than the rest, but you don’t know which one it is — and you don’t know if the different coin is heavier or lighter than the rest. Using just 3 weighings on the balance, can you identify which coin is the odd one out — and whether it’s heavier and lighter than the others?

That’s a small change but it makes the puzzle much harder. Give it a try, you’ll see.

(If you want the solution, click here.)

Originally published on December 30, 2014