Here are some pictures of my university students trying out the air pressure water rockets. We managed to get pictures right at lift off. I taught courses for university students who wanted to become teachers; I would take my students to work with children doing science in local elementary schools. Both the children and the university students were always fascinated by the rockets.
What do you notice about the water during the initial acceleration? Do the pictures help you visualize the event–(the equal and opposite forces, the conservation of momentum)?