To calculate Teresa's percent error in estimating the height of the giraffe, we start with the formula for percent error, which is given by:
\[ \text{Percent Error} = \left( \frac{\text{Estimated Value} - \text{Actual Value}}{\text{Actual Value}} \right) \times 100 \]
In this case, Teresa's estimated height of the giraffe is 18 feet, and the actual height is 16 feet. Plugging in these values, we first find the difference:
\[ \text{Difference} = 18 - 16 = 2 \text{ feet} \]
Next, we divide the difference by the actual value:
\[ \frac{2}{16} = 0.125 \]
Finally, we convert this to a percentage by multiplying by 100:
\[ 0.125 \times 100 = 12.5% \]
Thus, Teresa's percent error in estimating the giraffe's height is approximately 12.5%.