Here is another addition to the lessons on the analogy between a body organ/system and a machine. These work well for STEM lessons because they bring in two areas of science: Anatomy and Physiology/Biology and Physics/Engineering. For example, in this lesson, we compare the way electrons move along a wire to the way ions move back and forth along the axon of a neuron in order to send an impulse. It works for Physics where it talks about electricity and touches upon engineering too. It works for Anatomy and Physiology and Biology under the NGSS standards HS-LS1-2,3 where the emphasis is on the organization of body systems. The second page gives more detailed info on the making of an ion and the Action Potential steps.