Why do stars twinkle when planets usually do not?

Stars twinkle because their light passes through layers of moving air with different temperatures and densities. Those shifting air pockets bend the light slightly, making the star appear to brighten, dim, or change color from moment to moment. Planets usually twinkle much less because they appear as tiny disks rather than points, so the distortions average out more. The effect is strongest near the horizon, where light crosses more atmosphere. This same atmospheric distortion is why observatories are often built on high mountains or in space.