The fixed expression 笑得合不拢嘴 (xiào de hé bù lǒng zuǐ, 'laughing so hard one can't close one's mouth') is extremely common — basically the Chinese way to say 'grinning ear to ear'.
memory
合 (hé, to join/close together) + 拢 (lǒng, to gather/draw together) — a doubled meaning that emphasises the bringing-together motion. Picture two halves of a clamshell pulling closed.
