The Park City Zoo had a contest to see who could guess the weight of the zoo's newborn giraffe. The correct weight was 120 pounds. Emma's guess was 138 pounds. What is the percent error for Emma's guess?

To calculate the percent error of Emma's guess, we can use the following formula for percent error:

\[ \text{Percent Error} = \left(\frac{|\text{Guess} - \text{Actual}|}{\text{Actual}}\right) \times 100 \]

In this case, Emma’s guess is 138 pounds and the actual weight of the giraffe is 120 pounds.

1. First, we calculate the absolute error:

\[ |\text{Guess} - \text{Actual}| = |138 - 120| = 18 \text{ pounds} \]

2. Next, we plug this value into the percent error formula:

\[ \text{Percent Error} = \left(\frac{18}{120}\right) \times 100 \]

3. Now we calculate the fraction and multiply by 100:

\[ \frac{18}{120} = 0.15 \]

\[ 0.15 \times 100 = 15 \]

Thus, the percent error for Emma's guess is:

\[ \text{Percent Error} = 15% \]

Therefore, the final answer is:

\[ \boxed{15.0%} \]