To find the number of sides of a regular polygon given the measure of its interior angle, we can use the formula for the interior angle of a regular polygon:
\[ \text{Interior angle} = \frac{(n-2) \times 180}{n} \]
where \( n \) is the number of sides of the polygon.
Given that the interior angle measures 120°, we can set up the equation:
\[ 120 = \frac{(n-2) \times 180}{n} \]
To eliminate the fraction, multiply both sides of the equation by \( n \):
\[ 120n = (n-2) \times 180 \]
Expanding the right side:
\[ 120n = 180n - 360 \]
Next, we can rearrange the equation to isolate \( n \):
\[ 120n - 180n = -360 \]
This simplifies to:
\[ -60n = -360 \]
Dividing both sides by -60 gives:
\[ n = 6 \]
Thus, the polygon has \( 6 \) sides.
To confirm, we can check the interior angle of a polygon with 6 sides (a hexagon):
The interior angle of a hexagon is calculated as follows:
\[ \text{Interior angle} = \frac{(6-2) \times 180}{6} = \frac{4 \times 180}{6} = \frac{720}{6} = 120° \]
Since this matches the given interior angle, we can conclude that the polygon is indeed a regular hexagon.
The final answer is:
\[ \boxed{6} \]