A koan can be as short as a sentence. One such koan is when Ummon asked, “The world is such a wide world, why do you answer a bell and put on ceremonial robes?”

One thing I found attractive about studying Zen (including the koans) is that you don’t need to obey a bell or wear special robes. Yes, some have attained insight by doing these things, but that is not true Zen.

I think the true Zen student finds the bells and the practice in the everyday clothing and actions of a life.

Mumon’s comment on the koan:
When you understand, you belong to the family;
When you do not understand, you are a stranger.
Those who do not understand belong to the family,
And when they understand they are strangers.

The Gateless Gate, by Ekai, called Mu-mon, from sacred-texts.com

The Gateless Gate: All 48 Koans, with Commentary by Ekai, called Mumon