A master swordsman once strictly supervised his three sons in their daily training, demanding they hone their skills from dawn till dusk without fail.
One day, wanting to test their progress, the old master devised a clever trap. He left a door slightly ajar and balanced a pillow on top of it, so that the moment someone pushed the door open, the pillow would fall and hit them.
With the trap set, he sat in the main hall and summoned his sons one by one.
The eldest son had barely pushed the door an inch when he sensed something was amiss. He stopped, observed the situation carefully, and quickly spotted the pillow. Moving with quiet precision, he reached up on his tiptoes, removed the pillow, placed it aside neatly, and then went to greet his father.
The second son was less cautious and pushed the door wide open. However, his reflexes were sharp; as the pillow fell, he caught it mid-air before it could strike him.
The youngest son burst through the door with total indifference. When the pillow fell on him, he reacted instantly by drawing his sword and hacking it into two pieces.
The old master then gathered his three sons together. He held a formal ceremony for the eldest son, presenting him with his sword and declaring that his training was complete. He then encouraged the second son to keep practicing diligently, noting that his cultivation was not yet sufficient. Finally, he picked up a wooden ruler and gave the youngest son a severe thrashing, cursing him as a disgrace to the family.