Gender fluidity is the idea that a person's gender can change over time or in different situations. Just like how you might feel happy one day and sad the next, some people might feel like a boy or a girl one day and then the opposite on another day.
Think of it like a big jar of toys. Sometimes you might want to play with the dolls, and other times the toy cars might be more fun. Just like how your toy preferences change, some people's gender identity can change too.
Gender fluidity means that a person might not always feel like a boy or a girl, and that's okay! Some people might feel like both, or neither, or somewhere in between. It's important to remember that everyone is different and has their own unique experiences, including when it comes to gender.