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Imagine a tiny ant, a crayon, a juice box, and a guitar all lined up on a table.
They all have different lengths, but the metric system measures every single one of them using just one base unit: the meter.

The Metric Length Ladder

In the 1790s, French scientists designed the metric system around powers of 10 so that you never have to memorize awkward fractions—you just multiply or divide by 10.
Each unit is a specific fraction of a meter: a decimeter (dm) is 110 m\frac{1}{10}\text{ m}, a centimeter (cm) is 1100 m\frac{1}{100}\text{ m}, and a millimeter (mm) is 11000 m\frac{1}{1000}\text{ m}.
Notice how every step is exactly 10 times bigger than the step below it.
So how do you convert when you change from a large unit into a smaller one?

Going Down: Large Units to Small Units

When you measure the same object with a smaller unit, you need more pieces to cover the length, which means you multiply by 10 for each step down.
Let's convert 3.4 meters3.4\text{ meters} into millimeters. Moving from meters down to decimeters, centimeters, and millimeters takes 3 steps down.
Multiplying by 10 three times is the same as multiplying by 10×10×10=100010 \times 10 \times 10 = 1000, giving 3.4×1000=3400 mm3.4 \times 1000 = 3400\text{ mm}.