How to make a checkbox checked with jQuery?

Possible Duplicate:
How do I check a checkbox with JQuery or Javascript?

I'm trying to make a checkbox checked (or not) with jQuery.

My example HTML:

<input type="checkbox" id="test" name="test" />

Attempt at clearing a checkbox(doesn't work)

$('#test').val('off');

and at checking:

$('#test').val('on');

How do I control checkboxes with jQuery?