What's the difference between gender and sex?

Being a boy or a girl, for most children, is something that feels very natural. At birth, babies are assigned male or female based on physical characteristics. This refers to the "sex" or "assigned gender" of the child. Meanwhile, "gender identity" refers to an internal sense people have of who they are that comes from an interaction of biological traits, developmental influences, and environmental conditions. This may be male, female, somewhere in between, a combination of both or neither. 

Self-recognition of gender identity develops over time, much the same way a child's physical body does. Most children's asserted gender identity aligns with their assigned gender (sex). However, for some children, the match between their assigned gender and gender identity is not so clear.