It's dusk. The family who arrived earlier in the day and pitched their tent next to ours have just asked us if we'd like to join them for a short walk to see "something magical". We walk in the near-darkness down a grassy track to a lane with hedgerows on either side, the sea away to our right and the lights of Plymouth giving the clouds ahead of us an apricot-coloured underbelly, until something catches our eye. Two dots of greenish light in the hedge. Glowworms.