Electromagnetic responses in both time and frequency domains can be looked upon as weighted spatial averages of the nearby conductivity distribution. Using this concept, I analyze the simplest possible situation: a uniform half-space in the presence of uniform current sources. Both transient and harmonic measurements are equally effective in providing conductivity values for the half-space; while the conductivity values are the same, the averaging processes are not. To different types of measurements, even within the same domain, there correspond different types of sensitivity or weighting functions. Some of these functions emphasize the near-surface conductivity, while others emphasize the conductivity at depth. These and other features of the weighting functions explain the behavior of magnetotelluric sounding curves.