Forgetting what you have published

People usually do not ask: "explain this paper" they will read a paper and may ask a more specific question about a part they did not understand.

The professor may or may not be able to answer. If he wants to, he can look up the paper or the answer to the questions asked. His experience and knowledge will probably help him to find the answer faster than the person who asked the question. If he does not want to, or cannot answer the question because he forgot, does not have time, or for any other reason, the person asking is simply out of luck: they will have to ask someone else or do research themselves to find the answers they are looking for.

Nobody can remember all the details about 90 highly specialised scientific papers, especially if they involved lots of work by different authors that may have been done decades ago. Professors and other scientists are human beings: they forget things and make mistakes just like everyone else. This is not a problem.

One of the main reasons for writing a paper is that you no longer need to remember what's in it, because you can go back and read the paper.

One of the reasons I write papers is so that I don't lose insights that I worked hard to discover! I certainly cannot reproduce from memory the technical details of my papers except for the most recent ones (though I generally can remember the basic gist of them).

I actually write a huge number of notes to myself that I never publish or distribute. I put a lot of work into figuring out other people's work from my own point of view, and if I didn't write it down that effort would be wasted.

I periodically get emails with questions about my older papers, and I generally have to spend a bit of time re-reading them before I can give an intelligent answer. I've also had the following common experience many times. Someone (say, one of my graduate students) asks me a question. I'll get excited and we'll talk for a while without answering it. A few hours later, I remember that I once wrote a paper answering it!