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Materials
Tray, glass pipette of eye- or nose drops, empty plastic mineral water bottle (1.5 L), water.
Experiment
1. Fill the bottle with water like seen in Fig. 1. Place it into the tray.
2. Suck 1 cm of water into the pipette, let it float inside the bottle (Fig. 2).
    Tightly close the bottle (Fig. 1).
3. Press the bottle with both hands. Observe how the volume of air and of water changes inside the pipette (Fig. 3)
4. Press the bottle more vigorously. Observe more changes of the pipette (Abb. 4).
5. Look what happens when you stop pressing the bottle.
Observation
1. The volume of the air inside the pipette is reduced by pressing the bottle and more water enters.
2. The "diver" sinks.
3. By releasing the bottle the air volume inside the pipette decreases, water leaves it and the "diver" ascends.
Explanation
The pipette descends because its mass increases due to more water entering.
The pipette ascends because its mass decreases due to loss of water.
Figure 5 shows what happens with a closed plastic bottle full of air if a divers transports it 20 m below the water surface.
The huge pressure of water totally flattens it.