Fees for coaching

Yesterday I was trying to explain how an acid-base indicator works to my students. To explain colour perception, I asked "If you mix 10 drops of red ink and 10 drops of blue ink in a beaker of water, what colour will the solution be?" The answers were --- black, brown, green. Why do I even bother? Now you understand why I complain about coaching classes. When you join a coaching class, you have to deposit your common sense and common knowledge at the fees counter.