Answer:
During inhalation in mammals, air from outside enters the lungs. The lung volume expands. The diaphragm constricts with expansion of the chest wall.
The "volume" of the "thoracic cavity increases" thereby leading to decrease in pressure relative to the environment.
Breathing follows Boyle’s law which states that "pressure and volume" are "inversely related".
A "slight negative pressure" is maintained in the lungs so as to keep the airways open. The drop in pressure due to "increase in volume of thoracic cavity" creates a pressure gradient. Air then rushes in from outside into the lungs.