Final answer:
Goose bumps are caused by the contraction of arrector pili muscles, which pull on the hair follicles resulting in hairs standing up and potentially trapping more insulating air close to the skin.
Step-by-step explanation:
Goose bumps are caused by the arrector pili muscles contracting, which pull on hair follicles and make the hairs stand upright. When Sam feels cold, this is his body's automatic response to try and warm up by trapping more air under the hairs for insulation. This reaction is also connected to the sympathetic nervous system, which triggers these muscle contractions in response to cold or emotional stimuli, like fear. Goosebumps are caused by the arrector pili muscles pulling hair follicles into an upright position.
When the body is too cool, these muscles contract, moving hair follicles and lifting hair shafts, resulting in the appearance of goosebumps. This contraction of the arrector pili muscles is the cause of more air being trapped under the hairs to insulate the surface of the skin.