they just do it. it is an instinkt.
an organism react to a stimulus by its senses
They react in same way most animals do. They respond to any stimulus. The stimulus can be touch, smell, sight etc... Blue Whales are likely to not to respond to same things as humans such as touch but still react to stimulus. An example being swimming deeper when a boat is near by.
Thermoreceptors react to variation in temperature.
A stimulus
To React
does the same organism always react to the same stimulus at the same way
React to stimulus
running from the object
stimulus
A stimulus. ("stimuli" is the plural)
stimulus
stimulus