Ii-Molecules zithanda ukuhamba zisuka kwindawo apho zisuka okanye zininzi khona ezo ndawo ke sithi zii-high places concentration, ziye kwiindawo ezingekho kuzo nekuthiwa zii-low places of concentration, ngokuzihambela nje kwazo zingalawulwa nto. Umzekelo, kukho i-oxygen ethe chatha emphungeni xa kuthelekiswa naleyo isegazini, ngoko ke ii-molecules ze-oxygen ziyakuthanda ukuhamba zisiya egazini. Ngokunjalo, zingaphezulu ii-molecules ze-carbon dioxide egazini xa kuthelekiswa nezo zisemphungeni, ngoko ke ii-molecules ze-carbon dioxide ziyakuthanda ukuhamba ziye kungena emphungeni. .