Twain, Mark "Write what you know," said Mark Twain. This is one of those quotes that gets attributed to a lot of people. Mark Twain is the most likely candidate because he said it, but there are other people who have also claimed this achievement for themselves. If you write what you know, you're not going to be able to cover all aspects of life in your writing, but you will be able to cover more topics than if you wrote about something you didn't understand fully. Writing about things that you experience firsthand allows you to include specific details that can only be known by someone who has been through these things yourself. For example, when I write about science labs or math classes, I can describe how experiments work and which lab tools are needed for certain procedures. I've used this knowledge to create lab scenes in my novels that give readers a realistic idea of what it's like to be in a science class full of students working on different projects. There are times when you should definitely avoid writing about things that you don't know anything about. If you try to explain how electricity works or discuss different types of cancer with certainty, you're going to come off as ignorant and arrogant.