Four points are chosen at random on the surface of a sphere. What is the probability that the center of the sphere lies inside the tetrahedron whose vertices are at the four points?
1. **Choosing the first two points:**
   - Consider the first point \( P_1 \) on the surface of the sphere. Without loss of generality, we can place \( P_1 \) at the north pole of the sphere.
   - The second point \( P_2 \) is chosen randomly on the sphere. The angle between \( P_1 \) and \( P_2 \) is uniformly distributed between \( 0 \) and \( \pi \). The average angle between \( P_1 \) and \( P_2 \) is \( \frac{\pi}{2} \).

2. **Cross-section and third and fourth points:**
   - Consider the great circle that passes through \( P_1 \) and \( P_2 \). This great circle divides the sphere into two hemispheres.
   - The third point \( P_3 \) and the fourth point \( P_4 \) must lie on different sides of this great circle for the center \( O \) of the sphere to be inside the tetrahedron formed by \( P_1, P_2, P_3, \) and \( P_4 \). The probability of this happening is \( \frac{1}{2} \).

3. **Intersection point \( Q \):**
   - Let \( P_3P_4 \) intersect the great circle at point \( Q \). For the tetrahedron to contain the center \( O \), the triangle \( \triangle P_1P_2Q \) must also contain \( O \).
   - The location of \( Q \) is uniformly distributed along the great circle. The probability that \( Q \) lies in the shaded area (which ensures \( \triangle P_1P_2Q \) contains \( O \)) is given by the ratio of the angle subtended by the shaded area to the total angle \( 2\pi \).

4. **Calculating the probability:**
   - The angle subtended by \( P_1 \) and \( P_2 \) at the center \( O \) is \( \angle P_1OP_2 \). The average value of this angle is \( \frac{\pi}{2} \).
   - Therefore, the probability that \( Q \) lies in the shaded area is \( \frac{\frac{\pi}{2}}{2\pi} = \frac{1}{4} \).

5. **Total probability:**
   - The total probability that the center \( O \) lies inside the tetrahedron formed by the four points is the product of the two independent probabilities:
     \[
     \left( \frac{1}{2} \right) \left( \frac{1}{4} \right) = \frac{1}{8}
     \]

The final answer is \(\boxed{\frac{1}{8}}\).