One of the amazing things about humans is we have this ability to think about how we see ourselves. We call it our identity.
So many different things go in to making up our identity: What colour we like, what foods we prefer, whether we like pineapple on pizza, what activities, sports, careers we are good at and drawn to, who we are sexually attracted to and, perhaps most fundamental of all, what gender we consider ourselves to be.
You only have to watch and listen to young children playing and talking about other children and adults to understand how fundamental gender - boy or girl, man or woman - is to how we see the world.
But for some, the gender they identify as is different to their biological sex. It's not a new thing, but our growing understanding and acceptance of individuals' right to define their own gender identity is.
In part that's because we now understand how much damage crushing people's identity does.