Gentlemen, brace yourselves: once upon a time, you were a girl. Strictly speaking, when you were just an overgrown ball of cells in the womb you were female—but fortunately a few things happened along the way to ensure you were all man by birth. During the first few weeks of fetus development, the baby's internal and external genital structures are the same, regardless of whether you are ultimately going to be a boy or a girl. They have two sets of organs: one that can develop into the female sex organs and one that can develop into the male sex organs. The gonads will become ovaries or testicles, the phallus will become a clitoris or a penis, and the genital folds will become labia or scrotum. Which sex organs develop depends on the presence of the male hormone testosterone (in humans, the default sex is female). A person who is a hermaphrodite has both female and male genital characteristics and can also be called intersex people. This typically means that the organs on the inside are of one sex, while the organs on the outside are of another sex; for example, a hermaphrodite might have a penis and testicles, but inside, there are ovaries and possibly a uterus. In more rare cases, the chromosomes say a person is male or female, but the genitals say otherwise. They are not able to self-fertilize, because either their testicular tissue or their ovarian tissue wouldn’t function normally.