Brain teaser?

You have 12 pool . 11 of them are of equal weight. 1 is either heavier or lighter than the others. Your job is to figure out which ball is different and whether it is heavier or lighter. To help you there is a scale where you can put on either side. The scale will naturally tip toward the heavier side, but there is no digital readout or numbers on the scale. You may use the scale a total of three times and may put as many as you want on each side of the scale.