Stars twinkle for a fairly intuitive reason: The movement of the air in Earth’s atmosphere can momentarily dim a star’s light. This is why, says NASA, stars on the horizon seem most twinkly—“because there is a lot more atmosphere between you and a star near the horizon than between you and a star higher in the sky.”

But what about stars’ characteristic pointy star shape? The science behind that is surprising and has less to do with the stars or the Earth or with space than it does with us. Stars are shaped like stars, says Henry Seeing StarsReich in the Minute Physics video above, because of imperfections in the back of our eyeballs. Most intriguingly, says Reich, this biological explanation means that every one of us sees stars slightly differently.

Does anyone really see the star shape other than in drawings, flags or on the top of a Chtimas tree? Don’t we just see a bright dot of light?