Why do stars twinkle?

Stars are so far away that their light reaches you as just a tiny point. On its way down, that point has to pass through pockets of warm and cool air shifting around in the sky, kind of like looking at something through ripples in a pool. The air bends the light a tiny bit this way and that, and what you see is the brightness wobbling. That wobble is the twinkle.

Captured atmospheric refraction in plain language (warm/cool air pockets bending starlight). Skipped the technical term 'scintillation', the role of temperature gradients in the troposphere that drive the turbulence, and the point-source vs extended-source argument that explains why planets don't twinkle. Save the planet contrast and the technical vocabulary for a deeper conversation.

Stars twinkle (technically, scintillate) because Earth's atmosphere isn't optically uniform: temperature variations create cells of air with slightly different refractive indices, and these cells move around constantly with turbulence. Starlight reaches us essentially as a point source (even the largest nearby stars are less than 0.01 arcseconds across as seen from Earth), so the integrated refractive nudges across the atmosphere produce visible variation in apparent brightness and position. Planets are also small but are extended sources of about 5 to 50 arcseconds across, so the turbulence affects different parts of the disc independently and the variations average out, producing a steady appearance. Scintillation is worst near the horizon (longer atmospheric path) and at sites with warm thermals rising into cooler upper air.