function displayRandomBlock(MaxNum){
  var myDidYouKnowId = Math.round(MaxNum*Math.random())+1;
  if (myDidYouKnowId != null && myDidYouKnowId <= MaxNum ) {
    n = parseInt(myDidYouKnowId);
  }else{
    n=1;
  }
  document.getElementById('DidYouKnow'+n).style.display='block';
}