To someone not familiar with it, perhaps nothing is more puzzling about Chan than the practice of gong'an. Briefly, a gong'an (literally, "public case") is the record of a dialogue between a master and disciple. In a typical gong'an, a disciple asks the master a question and is given an enigmatic, sometimes strange and baffling answer that results in the disciple becoming enlightened.
For example, this is a very brief gong'an (many are longer and more complex):
At first, these encounters were collected along with commentary by masters, but around the Song dynasty (960-1276) Chan masters began to give them to disciples as objects of meditation. The disciple was asked to meditate on the encounter in order to penetrate its meaning by "investigating Chan." Basically, this means putting one's entire being into finding an answer to the gong'an without resorting to analysis, discursive reasoning, memory, or factual knowledge. To do this, the disciple sweeps from consciousness everything but the gong'an, so that it fills the whole mind. Night and day, while sitting, working, eating, even sleeping, the practitioner "holds" the gong'an uppermost in mind, immediately picking it up if "dropped." Effectively, this means putting down or putting aside all wandering thoughts, attachments, and preconceptions, leaving the mind with nothing left in it but the gong'an.
When the practitioner's mind is fully engaged in the gong'an, the intense desire to know the answer gives rise to a "doubt mass" which is not ordinary doubt but doubt that goes to the core of one's self-center. If the practitioner persists without interruption the doubt mass may grow to the point where it can no longer be sustained. At that moment, if the conditions are right, the doubt mass will dissolve (some say shatter) and the practitioner may experience an awakening, perhaps enlightenment. The power of the doubt mass is critical in this practice.
Eventually some Chan masters began using just a fragment of a gong'an in order to investigate Chan. As an example there is another famous gong'an involving Zhaozhou:
Of course this is a baffling answer considering that Buddhists see all sentient beings as having buddha-nature.
One of the great modern exponents of the huatou method was Master Xuyun (1840-1959), also known as "Empty Cloud." He described investigating huatou as looking for the head of a thought just before the thought arises. At the age of 56, when an attendant spilled hot tea on his hands, Xuyun dropped the teacup. At the sound of the teacup shattering, he became enlightened. Later he wrote this poem:
While Silent Illumination is a gentle and gradual kind of practice, gong'an and huatou are aggressive and energetic. However, both require a firm foundation of a stable mind, and both can be used as paths to enlightenment.