Lights and Shadows

Another day turning in circles, I cut the same doors in my chest, So you can see me with the top of your fingers, Hidden in my lungs, I take thin samples, Place them in Petri dishes and align them, They are the only proof you breathe like me, So you get why I schedule for you daily, This private tour of lights and shadows, Save puzzle pieces in my cells, And wait for you to knock on my sternum.