Subject: What physics is behind the 'stomp rocket'

I teach 8th grade science, and I recently purchased something called a stomp 
rocket.  It is a plastic rocket that you put on the end of a tube.  The tube is 
connected to a plastic/rubber bladder.  You stomp on the bladder, and the 
rocket shoots up to about 300 feet.  My question is what physics concepts could 
be taught using this stomp rocket?
Thank You,  Mike Miller

Re: What physics is behind the 'stomp rocket'