"Faeces, penis and baby, are all three solid bodies; they all three, by forcible entry or expulsion, stimulate a membraneous passage, i.e. the rectum and the vagina, the latter being as it were 'taken on lease' from the rectum." (Sigmund Freud, On Transformation of Instincts as Exemplified by Anal Eroticism)
In this essay, Freud suggests that a baby, a gift, a penis, money and faeces are all "ill-distinguished from one another" and "easily interchangeable". Common sense really.
