Stimuli can be
Stimulus is a detectable change inside the organism or in its surroundings which is responsible for evoking a specific reaction. The ability to be aware of the surroundings and give a response to the external stimuli is known as consciousness. It is a defining property of the living organisms. The stimuli is received by the sense organs such as eyes, ears, nose, tongue and skin are the sense organs in humans. The visible change or reaction which an organism produces as a result of applied stimulus is called response
The stimulus can be of three types: physical, chemical and biological. The example of physical stimulus is touch. The response to touch can be seen in Mimosa pudica (touch-me-not) as the leaves close by curling up. The example of chemical stimuli is the different taste of the different food items that you eat. The biological stimuli is seen when chemical messengers (hormones) interact with the cells.