Our human existence maintains life in an environment that surrounds us. Our lungs exchange gases. Our gastrointestinal system takes in liquids and solid; and in addition, our urinary tract along with this gastrointestinal tube eliminates the waste products. There are a few other things in this environment that affect us such as ambient temperature, atmospheric conditions, and various other items that make our existence an unique experience.
Now the basic gases are oxygen and carbon dioxide. The basic liquid is water. The basic solids are a host of items containing nutrients that provide the fuel. Everyday, everywhere, every time this exchange of gases, liquids, and solids keep us going. A common thread to our human existence it is.