Reflect on hard questions.

For example, Oxford and Cambridge applicants are often asked tricky questions in entrance exams that train their analytical thinking. Some of them are even published in books. So try to think about them yourself first, and then read the author's reasoning. I'll give you some examples, and you try to answer them: Will your bag ever be empty? An empty bag is not actually empty. In addition to dust, crumbs, scraps of paper, and germs, it is full of air, which takes up all the empty space. But what if you pump out the air? The bag would still be overflowing with the quantum energy produced by constantly appearing and disappearing particles and waves.