Why does the qater in a dropper not come out unless rubber bulb is pressed?

The water in a dropper does not come out until the rubber bulb is pressed because of the difference in air pressure. When the bulb is in its relaxed state, the pressure inside the dropper is lower than the atmospheric pressure, keeping the liquid inside. Pressing the bulb increases the internal pressure, forcing the liquid out through the nozzle. When the bulb is released, the pressure equalizes, preventing the water from flowing back in.