Twelve marbles
You have twelve seemingly-identical marbles. Eleven of them are made of glass and one of them is made of crystal, but you don’t know which one. All you know is that crystal is heavier than glass.
You have a scale you can compare marbles with: put any number of marbles on each side of the scale, and the scale will lean towards the heavier side.
By using the scale three times at most, determine which marble is made of crystal.