Oh, an oil-based pen in a place like this ...? I found a unique bento goods "Chakki" at Tokyu Hands, so I would like to introduce it.
The true identity of Chakki is a pen-type portable powdered tea sold by Asahara Kogyo. The tea inside is different according to the color of the main body.
The yellow-green pen is Shizuoka Ichibancha. When you remove the cap and press the pen tip, a cup of powdered green tea will pop out, so dissolve it in hot water (water is also OK).
The black pen is powdered oolong tea. It would be fun to prepare some and change them according to the mood of the day.
The price of teakki is around 500 yen. As a lunch box goods, of course, how about one at your desk?