1. **State the problem:** Lizzie has 24 feet of fencing to build a square pen. The perimeter $P$ of a square is given by $P = 4s$, where $s$ is the length of one side. 2. **Write the equation:** Given $4s = 24$, we need to find $s$. 3. **Solve for $s$:** Divide both sides by 4: $$\cancel{4}s = \cancel{4} \times 6$$ $$s = 6$$ So, each side is 6 feet long. 4. **Find the area:** The area $A$ of a square is $A = s^2$. Substitute $s = 6$: $$A = 6^2 = 36$$ 5. **Answer:** Each side of the pen will be 6 feet long, so the pen will have an area of 36 square feet.