The stimulus is a physical entity, which can provoke the response in the body of an organism. The hiding of the organism under the rock in response to the predator is and a type of external stimuli, as in this case, the organisms hide themselves inside the rock in response to the predator, which is present outside the body of the organism.
Answer: The reason why stem cell research is a big field in biology or even the world is because stem cells might be able to treat degenerative conditions as well as answering questions about human development.
