KS 3-4) To demonstrate the explosive nature of hydrogen, set up a zincacid generator and let pupils take a sample by filling an upturned test tube initially filled with water (keep sparks safely away from the production line). Pupils can then try various hydrogenair mixes and ignite them with lit splint. Different compositions give different "squeaky pops". If the teacher is brave, pop a 2:1 mix safely in a flask. Contrast the chemistry of hydrogen with helium (possibly using a lit splint into carbon dioxide "pretending to be" Helium). Both are light but only one is inert and only one is cheap; now ask why they ever used hydrogen and not helium for an "air-ship". Finally, watch the explosion using the link in the article.