1. Starch absorbs water: Sago contains a large amount of starch. After cooking, the starch will absorb water and expand, causing sago to become soft and rotten.

2. The role of enzymes: Sago contains amylase. After cooking, these enzymes will decompose starch and make sago softer.

3. Humid environment: If the cooked sago is placed in a humid environment, the moisture will further accelerate the process of the sago becoming soft and rotten.

4. Bacterial growth: If cooked sago is placed in an environment with a suitable temperature, bacteria will easily breed, causing the sago to deteriorate and become soft and rotten.