Why do the stars twinkle?

Simply, because we view stars through the earths atmosphere. Think of the atmosphere as a big pair of dirty glasses or windsheild at night.

The light of a planet has an area of positive value while that of a star is almost a point here on Earth, and the very thin light of a star is apt to be disturbed by interstellar matter or earth atmosphere.

Thanks to you both. I was out in the garden late one night having a cigarette and looked up for a change. And it got me curious!

The scientific name for the twinkling of stars is "stellar scintillation" (or astronomical scintillation)., btw.

[When I was in the Navy school, we had two days out on a school ship. There we had "to shoot stars" with the sextant. Since that time, the twinkling of some stars increased a lot!]