Clipping

1. \begin{scope}
   \clip (0,0) rectangle (4,8);
   %Start drawing
\end{scope}
   

\begin{tikzpicture}

    %color one half of a unit circle
    \begin{scope}
       \clip (0,0) circle (1cm);
       \fill[black] (0cm,1cm) rectangle (-1cm, -1cm);
    \end{scope}

    %fill heads
    \fill[white] (0,0.5) circle (0.5cm);
    \fill[black] (0,-0.5) circle (0.5cm);

    %fill eyes
    \fill[black] (0,0.5) circle (0.1cm);
    \fill[white] (0,-0.5) circle (0.1cm);

    %outer circle
    \draw (0,0) circle (1cm);

\end{tikzpicture}