Through observation, Dr. Montessori noticed that every child will become intrigued with certain things and want to do them as long as the child grows to certain age. She was puzzled: why do they want to do those activities in that period?

Biologist 杜佛利斯 from the Netherlands proposed what she achieved from long-term biological observation - all living beings have one "sensitive period" in their infancy, during this transient period, it will have very strong feeling for certain things. Montessori accepted the opinion of 杜佛利斯 and began explaining the various features and inclination shown in human's infancy with "sensitive period" phenomenon. 

The children in this stage will love their environment. As Montessori said, that is "children fall in love with environment." They incessantly look for the things for growth in the environment and pay all attention to it and neglect the surrounding other things. Furthermore, children's energy for contacting the environment in sensitive period is very intensive. Adults may be surprised at children's concentrated and repeated conduct of certain activities, but the children are still doing. 

Sensitive period includes: sensitive period of order, language, action, refining of sense organs, tiny things, social etiquette, etc.