Using fadein and append

If you hide the content before you append it and chain the fadeIn method to that, you should get the effect that you’re looking for.

// Create the DOM elements
$(content)
// Sets the style of the elements to "display:none"
    .hide()
// Appends the hidden elements to the "posts" element
    .appendTo('#posts')
// Fades the new content into view
    .fadeIn();