I receive regular emails from readers of my books, which is a delightful experience, and I always try to reply. But sometimes what they ask for is not very practical.
I have had the 'I have this assignment from school on light, will you write it for me?' type of email, for instance. To these I very gently point out that they can find out the information here and here, but it's up to them.
I also get emails and letters asking me to explain something in one of my books in a different way or in more detail. These I feel more sympathy for - at least part of me thinks that this implies that I didn't get it right the first time. If it's a quick query, I will do my best to answer it - but if it implies re-packaging the material of a whole chapter, say, I'm afraid I do give a fairly unhelpful answer.
I recently had one of these emails about a book I wrote 5 years ago, basically saying I don't get chapter 13 and 14 and as I like to understand each chapter as I go, can you explain it to me so I can read on? I replied that I could only really advise keeping going and hoping all will become clear. This is a common problem with science and maths subjects - you sometimes have to take bits on trust and go with the flow. It's certainly what I found when at university. In this case, we were dealing with infinity, a subject that is never going to have clear and absolute answers anyway.
Unfortunately, the email writer was not happy. He told me off for giving him a lame excuse. I find this quite upsetting. I wasn't trying to give a lame excuse - but the fact is, I can't write a new book for every reader to put the information across the way they want it. Many people have enjoyed that book - I know that from their emails. In this case, I failed. Half of me wants to go back and apologise - but I know it's the road to disaster. There are some conversations that aren't ever going to succeed.