Thread the string through the straw and stretch it
across a large horizontal distance. Inflate the balloon but
do not tie it. Tape the inflated balloon to the straw. Take
the straw back to one end of the string and release the balloon.
The balloon can be taken as a control volume. Air escaping the control volume at high velocity will produce
a net force (and therefore motion) in the opposite direction. The nice thing here is this is a good example
of a control volume with mass as a function of time.