Let's assume you were going to a department store for a certain list of items. You thought about what was in the store and composed your list. Something catches your eye. You resist. Something else catches your eye. Obviously we can't put blinders on in the store but that item is a need. You just did not know that you needed it prior to entering the store. That is the key. The item that you did not need prior to entering the store now you suddenly need.
Sometimes you will make the purchase and sometimes you won't. The rational that even though this item appears to be a need; you had no idea that you needed it prior to entering the store. "It is probably a want." Maybe you could adjust your list to fit it by not buying another item. If you do that you might not have really needed the other item if you were so ready to give up purchasing a listed item. So cross off that item and skip the want posing as a need.