So number one is mucus membrane. So inside our passageways, we basically have specialized types of cells known as goblet cells. And these goblet cells can produce and secrete a special type of sticky and slimy material or substance that essentially forms the mucous membrane. And this mucus membrane essentially lines our passageways. And when we breathe in the air, these harmful agents and harmful things, as they travel through with the air, they can essentially get stuck inside this mucus membrane. And if we move on to number two, we also have these hair like structures that can essentially move in a wavelike fashion that are found along the cells that line our passageways.