# TikZ doesn't fill when node coordinates are used

Can someone please explain me why the following code doesn't result in a fill?

And why, sometimes, I get such "very thin lines" artifact (that's visibile in my picture)?

\begin{tikzpicture}
\node (A) at (0,0) {A};
\node (B) at (1,0) {B};
\node (C) at (1,1) {C};
\fill[red] (A) -- (B) -- (C) --cycle;
\end{tikzpicture}


Here's what I get:

As pointed here, nodes have a shape and are not just pure coordinate. Your example works if you replace the fill line by real node coordinates:
\fill[red] (A.center) -- (B.center) -- (C.center) -- cycle;