\begin{itemize}
\item [(a)] Elementary examples: an arc, a circle, a noose, a figure eight,
a $\theta$-curve
\item [(b)] Acyclic graphs = trees.
\item [(c)] planability of graphs; the Kuratowski graphs $K_{3,3}$, $K_5$;
Whyburn's example of an open map from a planar onto a
nonplanar graph
\item [(d)] Problem of K\"onigsberg bridges; Euler's solution
\end{itemize}
