Endorphins, a known substances produced in brains is an opioid receptor agonist which the brain produce in the presence of pain stimulus.

• 如果你一连几天、一天几次进行实验：诱发某人身感疼痛，再用吗啡为他止疼，到最后一天，悄悄用盐水取代吗啡，你会发现，此时盐水也能止痛。