咧 almost always appears with 嘴 (mouth). 咧嘴 plus 笑 = a wide grin; plus 哭 = crying with the mouth pulled wide. The stretched mouth is the key image, in either joy or pain.
mistakes
咧 has three readings. The third-tone liě is the verb above. A first-tone liē appears in 大大咧咧 (happy-go-lucky) and 骂骂咧咧 (cursing under one's breath). A neutral-tone lie is a regional sentence-final particle, as in 好咧.
Left mouth radical, a small square picturing parted lips. It indexes 咧 in the speech-and-mouth family: 喊, 叫, 吃. Here it specifically marks the lips being pulled wide into a grin.
Right side 列 supplies the sound: liè drifting to liě in 咧嘴. The same phonetic powers 烈, 裂, 例. Faint semantic flavour too: when you grin your teeth show in a tidy 'row' under stretched lips.
