A tall glass, dropper, one thick-rubber balloon, rubber bands.
Fill the glass with water leaving one-eighth of it empty. Then fill the dropper also with water sufficient to hold it erect in the glass. Now float the dropper in the water of the glass and stretch a rubbr balloon on its mouth. To make it air-tight use rubber bands on top. That's all. Your preparations are done!
Just press the rubber top lightly with finger tip. What happens? The dropper, like an expert diver, dives towards the bottom! If you keep on pressing it, it will reach and touch the bottom. And on the contrary, as you take away your hand from the rubber sheet, the dropper jumps back to the water surface.
Under pressure, the air contracts more than water. So as the rubber lid is pressed down, the air is pressurised and this in turn puts pressure on the water as a result of which the water forces its way into the dropper and pushing its air,.fills it. So the dropper dives down. That's the secret of thi? extraordinary diver.