why not use jquery but use classie？

Sorry,i'm a fresh on html/css....
I found that jquery it's more easy use in controling DOM.
So why not use jquery?
Is classie.js lighter than jquery,so this demo uses classie.js?

Presumably because classie is much more lightweight than jQuery.

It would be easier if they just used jQuery for those of us already including that in our project, but it's not too much trouble to port it over to use jQuery instead--especially since Classie is no longer supported...