Okay, let's say you have three friends who all love different toys. One friend loves dolls, one friend loves trucks, and one friend loves building blocks.
Now, imagine that you really like your friend who loves building blocks, and you start to think that everyone who loves building blocks must be just like your friend. You might start to believe that all people who love building blocks are quiet, smart, and like to be alone.
But the truth is, not all people who love building blocks are the same! Just like how not all girls who love dolls are the same, and not all boys who love trucks are the same.
When we start to believe that everyone who likes the same thing is exactly the same, that's called stereotyping. It's not fair to assume that someone is a certain way just because they like a certain thing, or because they belong to a certain group like a race or religion.
The best thing to do is to get to know someone for who they really are, instead of making assumptions based on stereotypes.