678. The Substances Used for Writing

Shabbos 11:14

If a (right-handed) person writes with his left hand, the back of his hand, his feet, his mouth, or his elbow, he is not liable. If a left-handed person writes with his right hand, he is not liable and if he writes with his left, he is. An ambidextrous person is liable regardless of the hand he uses. If a child holds a pen and an adult guides the child’s hand, causing him to write, the adult is liable. If the adult holds the pen and the child guides his hand, the adult is exempt.

Shabbos 11:15

One is not liable for writing until he writes with a permanent substance - like ink, dye, vermilion, vitriol, etc. - on a surface that will preserve the writing - like hide, parchment, paper, wood, etc. One is not liable for writing with a temporary substance like beverages or fruit juice, or if he writes on a substance that will not preserve the writing like the leaves of produce. The same is true of erasing: one is only liable for erasing writing that would have been permanent from a surface that would have preserved it.