I was just thinking (it's painful, but I still do it sometimes) and happened to remember an incident that happened in my school back in the last century when I was still teaching. A friend of mine, who was a really good math teacher, was approached by a student with a long, very complicate equation. He asked the teacher if he could solve it. Not to be outdone, he worked on it all through his 25 minute lunch period. Just before the bell, he found himself multiplying by zero.
Howard