A very important quality of living things is the way they all respond, or react, to changes which take place around them. Anything which makes a living creature respond is called a 'stimulus' (the plural is 'stimuli'). A sudden flash of light in front of your eyes could be a stimulus —you would respond by blinking your eyes. Or touching a very hot object with your finger could be a stimulus— you would respond by jerking your hand away. Very much lower animals than man —even simple one-celled creatures like amebas— respond to stimuli. They move away from unpleasant places, where it is too bright or too warm, to find better surroundings.
Plants, also, respond to stimuli. For instance. they grow towards sunlight. But plants do not respond very quickly. They have no muscles for quick movement, and they do not need to move from place to place. They make their own food. and do not have to chase and catch it.
