Drawings of Characters with Open Mouths - Surprised, or Mad, or…?!
It may be hard to get at first glance, but the characters featured in this article have one thing in common: their open mouths are shaped like a chestnut. And pixiv has a tag for that.
栗みたいな口 (kuri mitai na kuchi, lit. “mouth shaped like a chestnut”) is a tag that's said to have originated from the adorable blonde character Luna Child (from Touhou Project), featured below. You might find the tagged characters looking a bit absent-minded, or even on the verge of saying something.