Think of a chemical change as being irreversible, like grass not returning to a seed. Ice can become liquid and then return to the ice. It is a chemical change because photosynthesis converts carbon dioxide and water into sugar (food) for the plant to grow and oxygen it can "exhale." Grass can't shrink after growing; any growth is considered a chemical change. A lawnmower, for example, is a physical change. This is a physical change because the grass is still grass even if it is cut shorter. Just like a sheet of paper ripped in half, it's still paper.