this is a reference to the member that invokes the current function...
then you can wrap it in the jquery function $() to select it just like you would another selector.
So setInterval calls a anonymous function so it is not invoked by a referencable member, so it defaults to the window object.
save the this context in a variable and then use it internally like this...
$(document).ready(function(){
$("#round").click(function(){
var clicked = this; //<----store the click context outside setInterval
setInterval(function(){
$(clicked).animate( //<----------use it here
{height: 250,
width: 150,
top:150,
left: 425},
{duration: 300}
).
animate(
{height: 200,
width: 200,
top:200,
left: 400},
{duration: 300}
);
}, 0);
});
});