function activate_teaser(obj) {
  
  var inactiveClassName = 'TeaserSmall';
  var activeClassName = 'TeaserBig';
  
  if(obj.className == activeClassName)
    return;
    
  var allElems = obj.parentNode.childNodes;
  obj.className = activeClassName;
  
  for(var i=0; i<allElems.length; i++) {
    
    if( allElems[i] !=  obj && allElems[i].className == activeClassName) {
      allElems[i].className = inactiveClassName;
    }
  
  }
  
}