Until recently–more recently than I care to admit–I would have said: “Words.”
Writers use words.
Writers do use words, but that’s not the right answer.
A well-known writer got collared by a university student who asked, ”Do you think I could be a writer?”
”Well,” the writer said, ”I don’t know. . . . Do you like sentences?”
The writer could see the student’s amazement. Sentences? Do I like sentences? I am 20 years old and do I like sentences? If he had liked sentences, of course, he could begin, like a joyful painter I knew. I asked him how he came to be a painter. He said, ”I liked the smell of the paint.”