Carol Martini, age 11, of Sarasota, Florida, for her question:

What is iodized salt?

Ordinary table salt becomes iodized when a trace of iodine is added by the manufacturers. This seems odd, because it is well known that the iodine bottle in the medicine cabinet is labeled as a poison. So it is. This germicide is a highly concentrated form of iodine and a small dose can be lethal when swallowed. However, the situation changes when this highly active chemical is scaled down to its individual molecules. On this scale, small traces of iodine are needed by the human thyroid gland.

In many regions, sufficient amounts of iodine are dissolved naturally in the water supplies. In regions where this does not occur, people suffer from thyroid deficiencies, and a throat swelling called goiter is common. The problem is solved very simply by adding traces of iodine to the table salt. This tricky job requires the professional skills of the manufacturers    because only a trace is needed and too much may be too much in a bad way.