Bread dough starts as a thick, heavy paste. A few hours later it has doubled in size and is full of holes. Something inside is making gas.

That something is baker’s yeast, a single-celled fungus called Saccharomyces cerevisiae. Each cell is only about 5 to 10 micrometres wide, so a pinch of dry yeast holds billions of them. Yeast is not a plant and not a bacterium. Fungi, like mushrooms and moulds, are their own kingdom of life.