Take any six pills and divide them into two groups of three each. Weigh putting the 3 pills on each side. If one side weigh's heavier than another, the heavier side consists of the poisonous pill. From the three pill in heavier side, weigh two pills. If one pill is heavier than another, the heavy one is poisonous pill. If both pill weigh same, the remaining third pill is poisonous. If all the 6 pill weigh the same, weigh the remaining two pill and heavy one is poisonous.