Shiota-sensei to Amai-chan - Vol. 1 Ch. 1 - The Right Way to Get Sore Muscles

[MangaDex]

 

[Simpleton]

It looks more like the stain of having an ink pen start leaking in your shirt pocket. (And since that's more of a seeping leak, it's a kind that is easier to overlook while it happens.)

So probably mangaka snatched an old story, but updated it to soy sauce since nobody uses ink pens anymore XD

 

[cksn]

so good
washes out the taste of so many beta-mc/egalitarian mangas