Transgender person here, there's two. See, when a fetus is developing, their brain developes to have certain structures and characteristics, which fuels behavior typical of their sex. For instance boys tend to be hardwired to fill the role of a provider and defender, where girls tend to be wired to be nurturing and motherly. On top of that, your brain is also programmed to know what your body should look like and to keep inventory on it. Like, your unconscious brain will go "ok do I still have 10 fingers and toes?" And your nerves send the message back to your brain that, yes, you do have 10 fingers and 10 toes. Your brain also does this with your private bits. This is how phantom limbs happen. A transgender person has a brain that is typical of the opposite sex, so they behave more like a typical member of the opposite sex. They also suffer from a mental disorder (that's not a bad word, don't get pissy) called Gender Dysphoria. Gender dysphoria is the brain's adverse reaction when the body doesn't match what the brain is programmed to have. This leads to things such as depression and anxiety. The best way I could describe gender dysphoria is that if I feel sensation on my breasts or downstairs, my brain has the reaction of touching a hot stove. The severity of gender dysphoria and its effects can fluctuate in severity, but it's been a persistent thing throughout my entire life. A transgender person's brain has trouble with this basic instinct of perceiving and recognizing your body parts. I don't identify as male, my brain is very literally male. All the genders besides male and female, they're all made up by kids who wanna be cool. That, or its a kid who's struggling with having gender dysphoria. Take gender fluid for instance, often times these are just kids who think that dressing girly and boyish means their gender has changed, and in a rare care it'll be some kid who goes "well I don't feel dysphoria all the time! So I must be gender fluid!" To which I say, my dysphoria isn't constant either, you're still male or female you moron.