The acceleration of gravity in Earth's interior is determined by the density distribution in Earth. A remarkable result is that the acceleration is approximately constant all over the mantle, which amounts to about 84% of Earth's volume. This result can be explained by a simple two-layer model of Earth, showing that the constancy of the acceleration in the mantle is a consequence of the particular size and density of Earth's core with respect to the size and density of the whole Earth. In other planets, with different mass distributions, the dependence of acceleration on depth could be very different.