window.onload=showhide

function showhide(){
  var spans=document.getElementsByTagName('span');
  
  for (var i=0;i<spans.length;i++){
    spans[i].onmouseover=show
    spans[i].onmouseout=hide 
  }
}
  
function show(){this.style.backgroundColor='transparent'}
function hide(){
  if(Math.random()<(.5)){
    this.style.backgroundColor='#000000'
  }
}