Consider this.

You have 3 boxes, labelled A, B & C.

You take the lid off box A, put a rubber duck in and shut the lid.
You take the lid off box B, don't put anything in and shut the lid.
You do not open box C.

You know the object is in box A, this is your known value, e.g. a string "rubber duck"
You know that box B has nothing in. This too is a known value, e.g. ""
You do not know if box C contains anything at all - it could be full or it could be empty. This value is not known. This is your null


Now imagine we add 3 new boxes in to the equation which have the same set up as before. We will call these A1, B1 & C1 respectively.


Now can you answer these questions?

Does A = A1?
Does A = B1?
Does A = C1?
Does B = A1?
Does B = B1?
Does B = C1?
Does C = A1?
Does C = B1?
Does C = C1?