One square centimeter is literally a square with side lengths of 1 centimeter:

One centimeter is equal to 10 millimeters, therefore this square also has a length of 10 mm on each side:

If you divided each side into millimeter long segments, you would see that one square centimeter is equal to 100 square millimeters (100 squares with side lengths of 1 mm).  Or simply,

\[1 cm^2 = 1 cm \times 1 cm = 10 mm \times 10 mm = 100 mm^2\]

To convert from square millimeters to square centimeters, you simply divide by 100.  For example,

\[1670 mm^2 = 16.70 cm^2 .\]

Tyler