Have students use a Sharpie marker to draw a line where the water is located. Ask the students to observe and infer what happens to the bulb when the bottle is firmly squeezed. Later in your presentation you could have students calculate the change in the bubble's size to cause it to accelerate down at 1 m/s2.
This is a simple make and take. It is amazing how many people have not done this. You will be surprised how many students will "dig this" activity.