with my nephews & nieces, we just used to touch the oven (when cold), go "ouch hot" while shaking fingers (like we'd burnt our fingers), each of the kids got to touch the oven when it was warm (not hot, but almost cold, but enough to feel the warmth) and we did the "ouch hot" with that, it took a couple of times and then the kids would stay away from the oven. My brother got a glass fronted fireplace when the two youngest were still quite small and they'd learnt "ouch hot" by then and would point to the fireplace and tell you "ouch hot".