Designers of commercial aircraft generally aim for stability. Imagine carrying a marble in a deep bowl. If you lurch this way or that, the marble will roll, but it will always return to the center. Instability is like balancing a pool cue on your finger: The further it gets away from straight up and down, the more it wants to fall. In the case of an airplane, the higher the nose gets, the higher it wants to go. If left unchecked, this could result in a situation called aerodynamic stall, where the wings lose lift and the plane starts to plummet.