In the following MWE:

```
\documentclass{article}
\usepackage{tikz}
\begin{document}
\thispagestyle{empty}
\begin{tikzpicture}
  \draw (0,0) to[in=180, out=0, looseness=.75] (3,3); 
  \draw[green] (0,0) --  (.25\textwidth,0); 
  \draw[red] (0,-3) to[in=180, out=0, looseness=.75] (.25\textwidth,0); 
  \end{tikzpicture}
\end{document}
```

the first two `\draw`s work, whereas the third gives this error:
```

<to be read again> 
                   \textwidth 
l.8 ...80, out=0, looseness=.75] (.25\textwidth,0)
                                                  ;
The control sequence marked <to be read again> should
not appear between \csname and \endcsname.

Missing character: There is no 0 in font nullfont!
! Extra \endcsname.
\pgfutil@ifundefined ...@ifx \csname #1\endcsname 
                                                  \relax 
l.8 ...80, out=0, looseness=.75] (.25\textwidth,0)
                                                  ;
I'm ignoring this, since I wasn't doing a \csname.
```

Why?