Can't declare Class field type of the other class

Can't declare Class field type of the other class

Why i can't declare Class field type of the other class? This gives me
C4430 error:
//Entity.h file
class Entity
{
public:
Box test;
};
class Box
{
public:
double length; // Length of a box
double breadth; // Breadth of a box
double height; // Height of a box
};