So I have twin 8 year old boys and we're exploring the world of things that go boom together. We've done the diet coke and Mentos thing and we've done the compressed air rocket thing.

They're not quite old enough yet (IMHO) for things that actually light on fire to explode or accelerate but I've got Estes model rockets on deck for next summer.

What I'm curious about is does anyone know of a fairly benign combination that can produce enough pressure to rupture a soda bottle but isn't flammable/highly caustic? The mentos/diet coke thing wasn't enough pressure and I don't know if baking soda and vinegar will do it. Is there another 'traditional' combination I've missed that I can share with my children and we can have fun with things that go boom?