Iodine tests for starch, a type of complex carbohydrate known as a polysaccharide.

Here's how it works:

* Starch is made up of long chains of glucose molecules.

* Iodine molecules can fit into the helical structure of starch molecules.

* This interaction creates a blue-black color that indicates the presence of starch.

So, if you add iodine solution to a sample and it turns blue-black, you know that starch is present.