Today I was speaking with a professor and he asked me, regarding fluids, what is pressure. I said force per unit area acting normal to a surface. He then asked what I meant by surface. I wasn't really sure how to respond. He also asked me what surface tension was. I said it is a force per unit length between molecules, like a cohesive force and that if we are given a square surface, surface tension acts to pull the sides of the square, ultimately minimizing surface energy. He then said the explanation he was looking for referred to cuts through a control volume, and whether pressure and surface tension depended on such cuts. Can someone finesse all of what I've said, whether I'm right or wrong, and explain the cuts he was referring to?