Problem    

You have 8 billard balls which all look identical. One is defective and weighs slightly more than the other balls. You cannot notice the difference by weighing them in your hands.

You also have a balance scale one which you can weigh the balls twice only. How can you work out which is the heavy ball?

Answer (Don't look til you tried working it out first!)

Weigh six balls, three per side. If these are the same weight then the heavy ball is one of the two you didn't weigh, weigh those two and you'll find the heavy ball. If one of the groups of three balls is heavier then the defective ball is in that group. Now weigh two of the three balls. If you defective ball is on the scale it will be heavier, if the scale balance then the defective ball is the one not on the scales.

Picture from: http://discover.edventures.com/images/termlib/b/balance_scale/support.gif