If the interest charged on $100 is 1 percent per year then every year the borrower must pay $1. If the $1 is not added to the amount owed then the borrower is paying simple interest. If simple interest of 1 percent is paid each year at the end of 100 years the borrower would have paid $100 in interest, $1 each year, and would still owe $100. If the $1 is added to the $100 the amount owed will increase to $101. 1 percent interest on $101 will be more than $1. This is called compound interest. Compound interest will cause the amount to grow each year. As the amount grows larger more is paid as interest each year and added to the total. If compound interest of 1 percent is paid on a loan of $100 each year the amount of interest paid will slightly increase and after 70 years the amount owed will have grown to $200 and the interest payment will have increased to $2. 