To iodize something means to add the chemical element iodine to it. The word comes up most often with salt: iodized salt is regular table salt with a tiny amount of iodine added.

Why would anyone add iodine to salt? Because our bodies need iodine to work properly, especially for the thyroid gland in our necks, which controls how we grow and how our bodies use energy. Without enough iodine, people can develop serious health problems, including a condition called goiter, where the thyroid swells up.

In the early 1900s, many Americans, especially those living far from the ocean, weren't getting enough iodine in their diets. (Seafood and seaweed contain lots of iodine naturally.) So in 1924, companies started iodizing salt, adding just a small amount of iodine to something nearly everyone uses every day. This simple change virtually eliminated iodine deficiency diseases in America.

When you see a container labeled “iodized salt” at the grocery store, you're looking at one of public health's quiet success stories: a simple solution that prevented millions of people from getting sick.