Left: 2x possible 可 (A nail 丁 into this opening 口 should be possible.), right: lack, deficiency 欠 (The mouth wide open ⺈ of the yawning person 人 is lacking [i.e. manners])
Mnemonic
You can sing twice "ka", but there is still a deficiency in your singing.
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!