BEJ241 10.3150/09-BEJ241 BEJ0810-013 18 Throwing balls on homogeneous surfaces Anne Estrade Université Paris Descartes anne.estrade@parisdescartes.fr Jacques Istas Université de Grenoble jacques.istas@imag.fr http://www.e-publications.org/ims/submission/index.php/BEJ/user/submissionFile/4077?confirm=e545ab61