What's the difference between a person's gender and biological sex?

What's the difference between a person's gender and biological sex?

Biological sex is a scientific classification. On the surface it's made by genitalia and external secondary sex characteristics; like breasts, or facial hair, or things you can see with your eyes but there is variation and there is gradation sometimes and knowing that is important. Science is showing that more and more. I think people need to start listening to that because it sort of explains what goes on with trans people a lot. Gender is funny because it's largely a sexual construct. The idea of what gender is has changed throughout the centuries and throughout cultures and societies. Gender is like blue is for boys and pink is for girls or women should wear dresses and men should wear pants.