Building systems can be compared to the biological systems of a living organism, such as a tree, to better understand how to make a building that supports the needs of users. Just like the roots of a tree, the building requires a foundation for support as the building increases in height. Water and nutrients flow throughout the tree, though we can’t always see these flows by just looking at a tree because the branches and foliage protect these systems. The walls, windows, and roof of a building protect the interior heating and cooling systems of a building similar to how a tree functions.