Leaven is a substance that makes dough rise by creating tiny bubbles of gas inside it. The most common leaven is yeast, a living microorganism that eats the sugars in bread dough and produces carbon dioxide gas. These bubbles get trapped in the dough, causing it to puff up and become light and airy instead of dense and flat. Without leaven, bread would be hard and heavy, like a cracker or matzo.

Bakers have used leaven for thousands of years. Before people understood yeast, they would save a piece of dough from each batch to add to the next one, creating what's called a sourdough starter. This old dough contained wild yeast from the air that would leaven the new batch. Today, most bakers use packaged yeast or baking powder, which work faster and more reliably.

The word also has a broader meaning: to leaven something means to make it lighter or more lively by adding a small but powerful ingredient. A serious discussion might be leavened with humor, meaning someone adds jokes to keep the mood from getting too heavy. Just as a little yeast transforms an entire loaf of bread, a small amount of something positive can transform a whole situation.