The correct answer to this open question is the following.
The real-life situation I encountered personally was to stereotype people according to the way they looked.
Unfortunately, I stereotype and prejudge people just because of their nationality. And I was so wrong.
Since I was a kid I have heard that immigrant people were not good. That they had a different way of thinking, different culture, and traditions that were not good for the American people.
So when I met this kind of people in school, I was on the defensive side. I was never rude to anyone, just put my "barriers" in front of me and tried to keep my distance.
However, one day during finals, I encounter some problems due to personal issues. I ask help from my friends, all American friends, and they never answered. To my surprise, these people from abroad were there for me and lent me a helping hand when I needed it the most. They treated me well as if I were one of them.
What I learned from that situation was that you never have to judge or stereotype people just because they are not like you. You always have to be open to know different kinds of people and welcome different ideas and cultures to learn more about people.
I promise to never judge people because I do not want other people to judge me without really knowing me.