How does the reaction between cornstarch and iodine demonstrate the presence of starch in a substance?

When cornstarch is mixed with iodine, the solution turns blue-black if starch is present. This color change occurs because iodine molecules bind to the starch molecules, forming a complex that reflects light in a way that makes the solution appear blue-black. This reaction is a simple and reliable way to detect the presence of starch in a substance.