A stimulus is anything that can trigger a physical or behavioural change in living things. The reaction of an organism to a stimulus is known as the response. The ability of an organism to respond to external stimuli is known as sensitivity. Some examples of stimuli and their responses are eating some food when we are hungry, a rabbit getting frightened, so it runs away, and contraction of the pupil of our eyes in bright light.

Reproduction is a process by which organisms give birth to their young ones, who are biologically similar to their parents. It is a fundamental feature of all the living organisms. It helps to maintain the continuity of the species.

Excretion is the process of removal of nitrogenous wastes and metabolic waste like excess salts, phosphates, sulphates, or any excess substances from the body. It plays an important role in maintaining the homeostatic condition of the body. Ammonia, urea, and uric acid are the major nitrogenous wastes that are produced from cell activity and digestion. Kidney helps in removal of waste from the body in the form of urine. The skin helps in the removal of wastes in the form of sweat. Lungs excrete carbon dioxide. The cells of the organism's body can stop functioning and can get very sick if these wastes are not cleared.

Respiration is a chemical process that takes place in all living beings. In this process, oxygen present in the inhaled air reacts with the digested food material in the body to produce energy, carbon dioxide and water. The taking in and giving out of air is called breathing which is a part of the process of respiration. Respiration can be categorised into aerobic or anaerobic, based on the requirement of oxygen.

Species is defined as a group of closely related individuals that are capable of interbreeding among themselves and producing fertile offsprings but are reproductively isolated from individuals of the other species. It is the basic taxonomical unit, ranking below a genus. Homo sapiens (humans), Felis catus (domestic cats), and Vulpes vulpes (red fox) are some examples of species.

Ecology is a branch of science that deals with the study of the interactions among organisms and between the organisms and their physical or abiotic environment. It is studied at various levels such as organism, population, community, biosphere and ecosystem.