Why do stars twinkle?

Stars twinkle due to atmospheric refraction of starlight. As starlight passes through Earth's atmosphere, it undergoes continuous refraction in a medium of gradually changing refractive index, bending towards the normal. This bending causes the apparent position of the star to fluctuate slightly, leading to the twinkling effect. The dynamic and non-uniform conditions of the Earth's atmosphere, especially near the horizon, contribute to this phenomenon.