No paper or no pen? Here is the simplest app.
"Tuya" is your drawing board which can be taken to any place, it can be used for drawing, drafting, creating brief strokes, showing something to others, working out math exercises and so on. It can even be used to help the deaf and dumb to communicate with others.
"Tuya" is super simple and convenient, it supports 5 different colors, 5 different line widths, and Save and Undo functions.