\documentclass[tikz]{standalone}\input{pre.tex}\begin{document}\begin{tikzpicture}
\draw plot[domain=0:350, smooth cycle] (\x:2+rnd*0.5);
\fill[magenta] (0,0) circle (2pt)
node [right] {$C(x_c,y_c,z_c)$};
\fill[blue] (0,1) rectangle ++(0.2,0.2)
node[above] {$m_i$};
\draw[->] (0.2,1.1) -- (1,1);
\fill[magenta] (1,0.9) rectangle ++(0.2,0.2)
node[above] {$dm$};
\end{tikzpicture}\end{document}