I think about it like this: When it is uncountable, it is aug known about, or an unmeasurable amount, and if you could quantify it, the amount is not the issue anyway. For instance: "We need paper." Do we need one sheet? of course not. We need an amount that will last. If that is 10 cases or just a packet does not matter - we just need some. "We are going to travel next week." Will we be gone for a day? Will we go several places? These are not the issue for now. Now what matters is that you know it is going to happen. I am baking bread. One loaf? 30 truck loads? Not the issue for now... Now lest try it with specifics... I need 10 pieces of paper at least. We will travel about 70 miles both ways. I have two loaves of bread ready; there will be more later. Now we have countable material. You can imagine the amount with out a question about the quantity.

expenditure, paper, suggestion, business, travel, experience, time, fact, accommodation, journey, correspondence, proposal, money, document I believe these can all be both countable and uncountable. The rest can't

What do you mean by countable and uncountable?