Mm, ninjutsu's a ninja technique that involves the molding of spiritual and physical chakra, which is a ninja's life force. When you mold it correctly, you are able to gain access to certain powers. In other words, I combined my spiritual and physical energy to make the water move.

[ Well, it's actually more complicated than that, and involves things like elemental affinities and what not, but he figures, as a basic explanation, this would be enough for a kid to understand. ]