why do humans frown reflexly when touching hot objects by mistake ??

Question

why don`t they just smile then??

Answer 1

this is due to a defense mechanism of body against other harmful factors it is called reflex reaction .
reflex reaction occurs due to neural signals of spinal cord to any part of the body which affected to that harmful factors like when u touch any hot object spinal cord send a signal to hands to pull back of that hot object
other examples of reflex reactions are when u hear a loud sound ur hear turns to it to see its source ,when u hit knees with a hammer it kicks.

Answer 2

the hot trigger the nerve in the skin and form a potential energy that gives sense of danger so the reflexion occur to prevent any injure. only person who had damage nerve can smile even a fire burn their body.

Answer 3

it's a defensive reaction, most animals do it: when they're scared or startled, they try to look as fearsome as possible.