Consider the two mutually tangent parabolas $y=x^2$ and $y=-x^2$. The upper parabola rolls without slipping around the fixed lower parabola. Find the locus of the focus of the moving parabola.
1. **Identify the fixed and moving parabolas:**
   - The fixed parabola is \( y = -x^2 \) with its focus at \( F_1(0, -\frac{1}{4}) \).
   - The moving parabola is \( y = x^2 \) with its focus at \( F(0, \frac{1}{4}) \).

2. **Determine the point of tangency:**
   - Let the point of tangency be \( T(\lambda, -\lambda^2) \).

3. **Find the equation of the tangent line at \( T \):**
   - The slope of the tangent line to \( y = -x^2 \) at \( T(\lambda, -\lambda^2) \) is given by the derivative \( \frac{dy}{dx} = -2\lambda \).
   - Therefore, the equation of the tangent line at \( T \) is:
     \[
     y - (-\lambda^2) = -2\lambda (x - \lambda)
     \]
     Simplifying, we get:
     \[
     y = -2\lambda x + \lambda^2
     \]

4. **Reflect the focus \( F_1 \) across the tangent line to find the new focus \( F' \):**
   - The focus \( F_1 \) is at \( (0, -\frac{1}{4}) \).
   - The line \( y = -2\lambda x + \lambda^2 \) intersects the x-axis at \( (\frac{\lambda}{2}, 0) \).
   - The reflection of \( F_1 \) across the tangent line can be found using the formula for the reflection of a point across a line. The reflected point \( F' \) will be the focus of the moving parabola.

5. **Calculate the coordinates of the reflected focus \( F' \):**
   - The reflection of a point \( (x_1, y_1) \) across a line \( Ax + By + C = 0 \) is given by:
     \[
     \left( x' , y' \right) = \left( x_1 - \frac{2A(Ax_1 + By_1 + C)}{A^2 + B^2}, y_1 - \frac{2B(Ax_1 + By_1 + C)}{A^2 + B^2} \right)
     \]
   - For the line \( -2\lambda x + y + \lambda^2 = 0 \) and point \( (0, -\frac{1}{4}) \):
     \[
     A = -2\lambda, B = 1, C = \lambda^2
     \]
     \[
     x' = 0 - \frac{2(-2\lambda)(-2\lambda \cdot 0 + 1 \cdot -\frac{1}{4} + \lambda^2)}{(-2\lambda)^2 + 1^2} = 0 - \frac{2(-2\lambda)(-\frac{1}{4} + \lambda^2)}{4\lambda^2 + 1}
     \]
     \[
     y' = -\frac{1}{4} - \frac{2(1)(-2\lambda \cdot 0 + 1 \cdot -\frac{1}{4} + \lambda^2)}{(-2\lambda)^2 + 1^2} = -\frac{1}{4} - \frac{2(-\frac{1}{4} + \lambda^2)}{4\lambda^2 + 1}
     \]
     Simplifying these expressions, we get:
     \[
     x' = \lambda
     \]
     \[
     y' = \frac{1}{4}
     \]

6. **Determine the locus of the focus \( F' \):**
   - As the moving parabola rolls around the fixed parabola, the focus \( F' \) traces out a path.
   - From the above calculations, the coordinates of \( F' \) are \( (\lambda, \frac{1}{4}) \).
   - Therefore, the locus of \( F' \) is the horizontal line \( y = \frac{1}{4} \).

The final answer is \( \boxed{ y = \frac{1}{4} } \).