A piece of Cron Nugget that is madd big, in comparison to all the other nugglets in the bong. This nugget is put in the bottom of the bowlsky so the grindulated cron doesn't fall into the bong water.
Stoner1: Yo man I just grinded up some bubonic chronic. May I put them in your bong?
Stoner2: Yes, but first, please allow me to place a stopper nug in the bottom