When we press the bulb of the dropper the air inside gets pushed out and hence the bubbles are seen and when we release the pressure on the bulb a low pressure is created hence the water fills inside the dropper
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When all the air is removed by pressing the bulb of dropper and pressure from bulb is released, the pressure of sir inside the bulb decreases. The outside pressure of water is higher than inside pressure of bulb. So water flows from high pressure to low pressure or to inside of bulb.
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