點解生果放幾日會自己熟?

因為令生果成熟的物質已經在果實中存在，即使摘下，反應仍然繼續，放多幾日就會變甜、變熟、變軟和變色。

原因是：【 Ripening 】
『 Ripening is a process in fruits that causes them to become more edible. In general, a fruit becomes sweeter, less acidic, less green, and softer as it ripens.
The life stages of a plant are influenced by hormones. An important plant hormone involved with ripening is ethylene, a gas created by plants from the amino acid methionine. Ethylene increases the intracellular levels of certain enzymes in fruit and fresh-cut products, which include:
Amylase, which hydrolyzes starch to produce simple sugars, and
Pectinase, which hydrolyzes pectin, a substance that keeps fruit hard.
Other enzymes break down the green pigment chlorophyll, which is replaced by blue, yellow, or red pigments. 』
http://en.wikipedia.org/wiki/Ripening

因為生果本身會釋放乙烯, 乙烯會使生果成熟.

把生果放入米缸內或密閉容器內, 生果釋放的乙烯積聚不散, 可使生果成熟速度加快.

因此人工合成的催熟劑多含有乙烯成份.