Stars twinkle due to the Earth's atmosphere. As light from a star passes through different layers of air in the atmosphere, it gets bent, or refracted, in various directions. This bending is caused by variations in air density and temperature, creating pockets of turbulent air. These changes cause the star's light to appear to flicker or twinkle when observed from the ground. The effect is more noticeable when the star is lower in the sky, as the light has to pass through a thicker layer of atmosphere, compared to when it's directly overhead. This phenomenon is known as 'tellar scintillation.'Interestingly, planets, which are closer and appear larger in the sky, don't twinkle as much because their light is less affected by atmospheric turbulence.