Why does calling a method in my derived class call the base class method?

Consider this code:

class Program
{
    static void Main(string[] args)
    {
        Person person = new Teacher();
        person.ShowInfo();
        Console.ReadLine();
    }
}

public class Person
{
    public void ShowInfo()
    {
        Console.WriteLine("I am Person");
    }
}
public class Teacher : Person
{
    public new void ShowInfo()
    {
        Console.WriteLine("I am Teacher");
    }
}

When I run this code, the following is outputted:

I am Person

However, you can see that it is an instance of Teacher, not of Person. Why does the code do that?