Probably with a pen and a piece of paper.
writer's block.
When a person knows how to read he can read and think things. Educated slaves are not wanted because they will want to improve their life and not be a slave. A educated slave can write letters and organize others against slavery. This is, of course, not wanted by the slave owner because he considers his slaves as property. Knowing how to think and read keeps people from slavery. When Hitler invaded Poland one of the first things he did was arrest the teachers and close the schools. He wanted to make the Polish people slaves and again a thinking educated person doesn't make a good slave.