Fundamentally, leavening is about introducing gasses of various kinds into baked grain pastes in an attempt to lightening them and make them easier to chew. I think we all know what you get when you try to bake up a simple mixture of water and flour without a leavening agent: a cracker. Hardtack is what people used to call crackers of the really, really thick variety. But while crackers and hardtack aren’t without their charm (they also keep very well as long as they’re kept dry), they aren’t all that interesting to eat. Breads and pastries on the other hand, are a whole different ball game. However it’s worth remembering that without things like yeast, chemical leaveners and dough layering techniques, we’d have very little bakery that we wouldn’t have to worry about cracking our teeth on.