The two ways in which plants lose water are transpiration and spitting water. Plants use this to regulate their osmotic pressure and body temperature. This is an important physiological activity of plants.

When the temperature is not very high, the stomata are opened and water is transpiration through the stomata; When the sun shines directly at noon or the temperature is too high, in order to avoid excessive water loss, the pores will be closed, and the water vapor that has not evaporated in time will condense on the cuticle of the leaf surface and flow out from the top of the leaf, forming the effect of spitting water.

It may also be the secretion of willow cicadas on trees, or the "honey dew" secreted by aphids.