How to add a variable into a class which is not defined in the class. I want to add it after creating the class.

Question

class Car
{
string color = "red";
}
After creating the class I want to add some more variables.
For example :
int weight =256;
int speed= 234;
I want to add these variables in the car class. How can I do it?

Answer 1

Can I add those variables while creating a object?

Answer 2

But what's a problem to declare them directly inside the class?