The sturdy rice sack is a type of sack, usually best known as punching bags. Inside the bag is tons of rice and is very sturdy! This punching bag can give you lots of XP, also this doesn't give you any money!

Practice and keep trying. When your ninja masters it, he will do better than he did last time when he was a beginner at the sack! Soon, you will find Kira.

Buy all the sacks(aka punching bags) and master them. This will improve his punching and kicking ninja skills . He will improve when he was such a beginner and so clumsy! You will see big differences.