Metric System
The metric system is a decimal system of measurement in which units are related by powers of ten, making conversions simple.
Definition
The metric system is a way of measuring in which every unit is $10$ times bigger or smaller than the next, so changing units is as easy as moving a decimal point. Base units (meter, gram, liter) are scaled by prefixes representing powers of ten, such as kilo- for $1000$, centi- for $1/100$, and milli- for $1/1000$, and lab quantities of any size can be written with a convenient prefix. Its power lies in one prefix set applied uniformly to every unit and in the historical linkage of units (a liter as a cubic decimeter, a gram as the mass of a cubic centimeter of water near $4^\circ\text{C}$), which makes length, volume, and mass interconvertible without constants. Science everywhere uses it, and the United States uses it in science, medicine, and manufacturing while keeping customary units for daily life.
Example
A bean plant $8.5\ \text{cm}$ tall is also $85\ \text{mm}$ tall. A cylinder holding $250\ \text{mL}$ holds $0.250\ \text{L}$, a $350\ \text{g}$ cart is $0.350\ \text{kg}$, and a $1.5\ \text{m}$ ramp is $150\ \text{cm}$ or $1500\ \text{mm}$, each a decimal shift rather than a multiplication table. A $2.0\ \text{L}$ bottle of water has a volume of $2000\ \text{cm}^3$ and, at $1.00\ \text{g/cm}^3$, a mass of $2000\ \text{g} = 2.0\ \text{kg}$, whereas the same reasoning in customary units needs a memorized constant like $8.34$ pounds per gallon.
Key Insight
Inches to feet is $12$ and feet to miles is $5280$, but metric is always $10$, $100$, or $1000$, which is why science uses it. The system links length, volume, and mass through water: $1\ \text{cm}^3$ holds $1\ \text{mL}$, and $1\ \text{mL}$ of water has a mass of about $1\ \text{g}$. The Mars Climate Orbiter was lost in 1999 because one team used pound-force seconds and another used newton-seconds; unit systems are not a formality, and a mismatch can destroy the experiment.