It seems an actuary was retained by a major airline to do a study on the statistical chances of a bomb being on an airplane. Before the study, this particular actuary never flew on planes due to his personal fear of a bombing.

After the study, his associates noticed he now flew everywhere. One of his colleagues asked about this marked change in behavior.

The actuary replied that he had determined in his research that the statistical chance of a bomb being on an airplane was very small, though not small enough to relieve his anxiety. But then, he said, he determined that the probability of TWO bombs being on the same airplane was infinitesimal.

Hence he's now quite comfortable flying on airplanes, because he makes it a point to always carry along one bomb!