1. **State the problem:** The friends have picked up trash in some shaded squares of a park divided into a grid. They want to say they have picked up trash in exactly $\frac{1}{2}$ of the park. 2. **Understand the fraction:** The fraction $\frac{1}{2}$ means half of the total number of squares in the grid. 3. **Check current shaded squares:** The problem states the shaded area already represents $\frac{1}{2}$ of the park. 4. **Interpretation:** Since the shaded squares already cover half the park, the friends do not need to pick up trash in any more squares. 5. **Conclusion:** The correct action is: **Nothing. They have already picked up trash in $\frac{1}{2}$ of the park.**