>You have 12 billiard balls.
>
>One of them is either lighter or heavier than the others.
>
>You can weigh the billiard balls 3 times on a balance scale.
>
>You need to find out which ball it is and whether it is heaver or lighter.

Here is a similar problem:

There are 6 balls: 2 red, 2 blue and 2 white. One of the balls of each color is slightly lighter than the second ball of the same color (you can name these two weighs as A & B). You need to sort out all the balls. You can weigh them only 2 times on a balance scale.