There are eight pills. They are all the same size and color. One pill weighs slightly more than the others and it is poisonous. You have a balanced scale and can only use it twice. How can you find the poisoned pill?
Take any six pills and divide them into two groups of three each; if the two groups weigh out evenly, weigh the remaining two pills to find the poisoned one.
If the two groups of three do not weigh out evenly, simply take two out of the three pills from the heavier scale and weigh them (using the scale a second time).
The two pills will either weigh the same, in which case the remaining pill is the poisoned one—or one of the two pills on the scale will weigh more.