The Sai is a 3-pronged weapon used in close combat like a knife. Though the design my very from nation to nation it still maintains a central long spike with 2 shorter spikes on either side which helped to protect the hands of the individual wielding it. Now some show that the 2 shorter pronges may have 1 facing the same direction as the central spike and 1 facing down towards the handle, or that both short spikes face the same direction as the longer central spike.
Now like many of the other weapons that the ninjas would employ, this was also a tool that was used by the peasants in everyday operations. Its exact use can not be certain, some believe it was used for digging holes for rice, or measuring the stalks of plants. Others believe that this was used to lock cart wheels so the cart would not roll away. Either way in the hands of a trained martial artist this was a deadly weapon.
The side prongs allowed the wielder to block or catch the blade of a long sword and with a twist of the wrist it may have been possible to crack or break the sword blade. Though the central spike was not sharp as a sword blade would be its long tapered design would allow it to be punched into an enemy causing death or severe injury. On the back side of the Sai was a ball or knuckle which allowed for a solid blunt force that was able to break or crack bones in your enemy giving you a chance to escape or disarm and kill your enemy.
So, given the commonplace of this agriculturaltool/weapon a trained ninja would be able to carry it around and not raise suspicion from gaurds and soldiers. This allowed ninjas to travel the countryside well armed and also able to protect themselves or kill their enemies at anytime.
