function show(ev) { 
    var element = ev.target;
    element.style.backgroundColor = "silver";
  }
  
  
  function addAll() {
    var item;
    var checks = document.getElementsByTagName("span"); 
    for (item of checks) {
      if (item.getAttribute("class") == "check") {
        item.addEventListener("click", show, false);
      }
    }
  }
  window.onload = addAll;