We had a contest called the Flask race to see who could fill the Erlenmeyer flask to the 200ml mark the first. On my command, the 2 students poured the water and 1 flask quickly filled. The other flask appeared stuck, with little or no water going into it. We repeated this several times until the kids caught on that I had notched one of the stoppers so that the displaced air could escape and the water could go into the flask. The flask without a notch didn't permit any air to escape, thus the forces were balanced and the water couldn't get into the flask. This showed that air takes up space.
We next weighed a balloon to show that air has weight.