Black Ink, Ink
You use black dirt to make black ink. That's what you use to write in black!
Make sure the dirt you're using to make black ink is really, really black. Like the kind you find around volcanoes or something.
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すみ
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Be careful that you don't spill black ink on anyone or anything. If you do, you have to own it. Imagine spilling black ink on someone, and when they confront you, you yell, "Yeah I got black ink on you, so sue me (すみ)!"
Imagine yelling "SUE ME!" at someone's face, which is covered in the black ink you spilled there.