Christopher Murphey, age 9, of Bala Cynwyd, Pa., for his question:

WHO INVENTED YEAST FOR BAKING?

The very first homemade bread was flat and heavy, more like crackers. This was ages ago, when our ancestors lived in caves. Light, puffy loaves, full of spongy holes, were impossible until people discovered yeast. This also happened long ago, so long ago that nobody remembers when. But almost surely the first bread baking yeast was discovered by accident.

A spoonful of yeast is really a whole multitude of tiny plant cells, too small for our eyes to see. Many types float around in the air and we never notice them. Ages ago, somebody left a doughy paste out in the air  and some of these drifting yeast cells fell onto it.  As usual they multiplied and made spongy holes in the bread.  This, no doubt, is how yeast was discovered. But it happened so long ago that nobody knows who made the discovery.