吃馒头时，越嚼感觉越甜，是因为

根据短文内容，完成下列各小题。

    It's four thirty in the afternoon. There are many students on the playground. Jim and his friends are playing basketball. Li Lei and other boys are playing football. Miss Gao is watching the games. She is looking after the clothes, too. Where are the girls? They are over there. Some are running, others are jumping. The twins are on duty today. They are in the classroom. Lucy is cleaning the window, Lily is sweeping the floor. They are working hard. They are tired(疲倦的) but happy.

控制变量是科学探究的重要方法之一．小华同学实验中发现：把质量相等的不同物质放入同样一杯水中，有的溶解快，有的溶解多，有的溶解又快又多．是哪些因素影响着物质溶解的快慢与多少呢？

请你参考下表硝酸钾的溶解度随温度的变化关系，以硝酸钾为例，选择一种可能影响硝酸钾溶解快慢的因素设计实验进行探究：

①影响因素：____ 实验中，控制不变的因素是____ ．

②实验方案：____ ．