Presidential humor is only a half-step from dad humor, which in the world of comedy is considered light treason. But we’ve tried to find at least one zinger from every U.S. president, to mark President Obama’s final set of stand-up comedy at his eighth White House Correspondents’ Association dinner on Saturday. Some zingers have less zing than others. Some are flat, some are apocryphal, some are just threats and some, in the case of Warren Harding, are quaint, loving references to the chief executive genitalia. But they are presidential, by definition, and therefore funny, by acclamation.