Final answer:
Green onions taste spicy raw due to sulfur-containing compounds, but heating changes these compounds and brings out the onions' natural sugars. Peppers are perceived as hot due to capsaicin affecting calcium channels, and individuals' heat tolerance can affect their perception of spiciness in foods.
Step-by-step explanation:
Raw green onions have a spicy taste mainly due to sulfur-containing compounds they possess, which can cause a sensation of spiciness or sharpness in their raw state. When green onions are heated, these compounds undergo chemical transformations, which reduce or eliminate the spicy flavor and allow the natural sugars present in the onions to dominate, leading to a sweeter taste. This is similar to how peppers, which contain capsaicin, taste "hot" because capsaicin opens the same calcium channels as warm receptors in our mouths, leading to an excess stimulation that gives the perception of pain or heat. Different individuals may find peppers to be less painful or hot depending on their tolerance for heat, which is due to differences in sensory receptors or prior exposure to spicy foods.