... And what is the principle? It's the interest This is the money after the interest has been added. This is the original money amount loaned. No one knows

The principle is the original amount of money loaned or invested, before any interest is added. It is the base amount on which interest is calculated. For example, if you borrow $1,000, that amount is the principle. If you earn interest on an investment, the interest earned is calculated based on the principle.