City University of New York
Exploring Life: Response to Stimuli
A good explanation of the concept that living things respond to their environment, or stimuli in their environment. The site gives real life examples.
Khan Academy
Khan Academy: The Biology of Awareness
An article discussing how organisms responds to their environment which is one of the characteristics of life. Learn how every organisms has awareness from the single-celled to humans.
CK-12 Foundation
Ck 12: Life Science: 7.13 Tropisms
Learn how plants respond to different environmental stimuli.