This is a super cute idea. To have a chalkboard, a piece of chalk, and a traditional Japanese chalkboard eraser (made of wood) all on your cell phone strap. You can write yourself reminder notes or doodle on it while you’re on the train.
FYI, the doodle on the image above is a "henoheno moheji." Its legacy is kind of like the smiley face in the US–except it’s a little bit deeper than dots and lines–it consists of seven hiragana letters which, when put together in this particular arrangement, look like a face! I personally have not drawn a henoheno moheji in probably over a decade. So I’m going to get off the damn computer now and go draw one.
Get your chalkboard here.