Kayu refers to a Japanese rice gruel - a watery, soft cooked rice that resembles oatmeal and porridge. It is an appropriate dish for making used of left over rice. Since Kayu can easily be disgested, it is usually served to people who are sick. This is also a traditional practice in my home country, the Philippines, where we usually serve "plain Lugaw "(only with a bit of salt or sugar) to sick persons, also because it is easily digested.