Explain why planets do not twinkle but stars twinkle.

The stars, planets, even the Sun and Moon twinkle, all in varying amounts. Anything outside the atmosphere is going to twinkle. Stars appear as a single point in the sky, because of the great distance between us and them. This single point can be highly affected by atmospheric turbulence. Planets, being much closer, appear as disks.
We can’t resolve them as disks with our eyes, but it still averages out as a more stable light in the sky.