You need to increase the volume of your thoracic cavity so that the pressure inside is lower than the pressure in the atmosphere. The external intercostal muscles sitting between your rib cage can track them and flare your rib cage up and out, increasing the thoracic volume. Your lungs have two membranes called the visceral pleura and parietal pleura which stick to each other - they're connected by a membrane known as the pleural cavity. This means when you take a breath in it's caught in Spire. It's pressure gradient because it's higher out than in. Air just rushes in. That's essentially how you cough or sneeze really.