Riddle: My veins of green may go unseen, Til on the ground I lay around. What am I?
Answer: "Leaves" - Reasoning: Leaves contain veins to transport water through the leaf, and as people would need to be up close to see them. Hence, these veins go unseen as they spend most of their time up in trees attached to branches. You only really get to see them when they fall to the ground.