Think of the square as a circle meaning 'OK'. (Curves generally get squared off as in 口 kǒu 'mouth' and 耳 ěr 'ear'.)
Common words: 可以 kěyǐ 'can', 可爱 kěài 'cute'
Common word: 哥哥 gēge 'elder brother'
Common word: 唱歌 chànggē 'sing a song'
Originally 欠 qiàn was a picture of a man yawning his head off, but now it means 'to lack'.