Both millimeter and centimeter are metric units used to measure length. "Milli" is prefix that means one thousandth. One millimeter, therefore, is one thousandth of a meter. Similarly, "centi" is a prefix that means one hundredth. One centimeter, therefore, is one hundredth of a meter. Its meaning can also be gleamed from two crawly creatures. A millipede literally means "thousand legs" and centipede means "hundred legs."
Converting from millimeter to centimeter is a piece of cake. One centimeter has 10 millimeters. To covert, move the decimal one place to the left and voila! The millimeter measurement is now in centimeters. To covert from centimeter to millimeter, multiply by 10. That is, just add a zero at the end.
Things You'll Need
 Ruler

Tell your students to draw a scale and mark centimeters and millimeters in order to get a sense of their proportions.

Pass a ruler around the class that has millimeters and centimeters clearly marked. Instruct them to count the number of millimeters in one centimeter.

Ask students to convert their heights into centimeters and into millimeters. This exercise can be extended to express any small lengths in large units and viceversa. The student's height in kilometers will appear much smaller  and amusing  than in millimeters.

Demonstrate to the students the uses of centimeter and millimeter. For example, millimeter is used to measure small lengths such as the thickness of a guitar string, while centimeter is the width of an average blouse button.
References
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