New (handles https from xkcd - explainxkcd does not handle https)
javascript:(function(){document.location = document.location.href.replace(/https?\:\/\//, 'http://explain')})();
Old
javascript:(function(){document.location.host = 'explain' + document.location.host})();
 -
                
                javascript:(function(){document.location = document.location.href.replace(/https?\:\/\//, 'http://explain')})();
  
 -
                
                javascript:(function(){document.location = document.location.href.replace(/https?\:\/\//, 'http://explain')})();