A fraction can be modeled by a whole divided into equal parts.

The sum of two fractions can be represented using fraction models.

$\frac{1}{8}$ can be represented by a grid divided into eight equal parts with one part shaded.

$\frac{2}{8}$ can be represented by a grid divided into eight equal parts with two parts shaded.

$\frac{3}{8}$ can be represented by a grid divided into eight equal parts with three parts shaded.

So, the model below represents the number sentence.

The first fraction circle is divided into three equal parts with one part shaded.

So, the first fraction circle represents $\frac{1}{3}$.

The second fraction circle is divided into three equal parts with one part shaded.

So, the second fraction circle represents $\frac{1}{3}$.

Combine the shaded pieces into one fraction circle.

The sum of the two fractions is represented by a fraction circle divided into three equal parts with two parts shaded.

So, the sum is $\frac{2}{3}$.

Therefore, the number sentence $\frac{1}{3}+\frac{1}{3}=\frac{2}{3}$ can be used to find the sum of the fractions.