oops concept query

Question

This is my oops query.

This is my base class

public class BaseClass
{
   public int i = 10;
   public int x = 30;
   public string str = "Hello";
   public virtual string Hello()
   {
     return "Hello of base class called";
   }
}

This is my child class

public class ChildClass : BaseClass
{
    public int i = 20;
    public int z = 90;
    public override string Hello()
    {
       return "Hello of child class called";
     }
}

I've since discovered that the code below is correct.

BaseClass baseObject = new ChildClass();

And only BaseClass members are displayed when I type baseObject.

Can someone provide me any advice regarding a scenario when a developer must execute BaseClass baseObject = new ChildClass()?

Why can't I access my child member variables through this baseObject if my BaseClass object contains a reference to my child class object?