<p>Hello</p><p style="display: none">Hello Again</p>

$("p").toggle()

<p tyle="display: none">Hello</p><p style="display: block">Hello Again</p>

<p id = "tt">Hello</p>
<input type="button" value="變換" id="b">

var i = 0;
$("#b").click(function(){
$("#tt").toggle(i++%2==0);
});

<p style="display: none" id = "t">Hello Again</p>

$("#t").toggle("slow",function(){
alert("123456");
});