'Venus' or 'Penus'

Yesterday I was teaching my son and sudenly , he shocked me with his rhyming knowledge and simplicity. I was teaching him about the planet and told him that a planet is 'Venus' . I was not finished when he shouted 'what Penus ?' . I said no it is Venus and then he said but it rhymes with 'Penus' ...I simplye could not do anything but smile...