Explain why the planets do not twinkle.

Explanation

Planets are much closer to the earth relative to stars. Therefore, planets appear bigger than stars. Stars are far away from the earth. Therefore they appear smaller.
So, stars can be considered as point-sized sources and planets can be considered as a collection of a large number of point-sized sources of light, i.e., extended sources of light.
As planet is a collection of a large number of point-sized sources of light, the total variation in amount of light entering our eye from all the individual point-sized sources averages out to zero, thereby nullifying the twinkling effect. That is why planets do not twinkle.