The CoG is the point which an object will be perfectly balanced. If you were to stick a needle upright on a table, and then take an irregular piece of card, rest it on the needle and move it around...
"Center of mass" and "Center of gravity" are properties of a quantity of mass in a stable shape. They don't exist where mass is zero, or where the boundaries of the shape are unknown. So it seems...
I assume you are talking about a model rocket. Center of pressure needs to be below center of gravity in order for the rocket to fly straight. Mathematically, the rocket will tilt around the center...