Metal meets nature. Everything else doesn't matter anymore. 
Leaving a leaf on a metal board, outside, for two to four days, will leave a temporary mark. The image is continuously changing, ah the ephemerality of nature, of life. After a few hours, the image will start to fade out, "the leaf"  will be eaten by the rust, and so, the cycle continues. There's a great metaphor that we all have to keep in mind.