I started by reviewing with them what they already understand about an electrical circuit. Drawing a simple circuit on the board I asked them to tell me what was going on. They quickly identified the closed circuit. I explained that if we were working with actual k-12 students we would have several lessons on electrical circuits perhaps using LED Lights LED Lights or Squishy Circuits Fun with Squishy Circuits .
Then I introduced Makey Makey by showing them the Makey Makey promo video.