Cynthia D. Custis, age 9, of Newport News, Virginia, for her question:

Where does mildew come from?

A patch of mildew often appears on a forgotten slice of bread, looking like a bad bruise. Actually, it is a weird little plant, made mostly of pale threads. It is a fungus plant, related to the mushrooms. None of these plants has any greenery to make its own food. So it must have rich foods from other plants and sometimes from animals. It spreads from place to place by tiny seedlets called spores.    .

The minispores of a mildew plant are too small for our eyes to see. Sometimes they float around from leaf to leaf on films of dewy moisture. Often the tiny spores float around in the air  but we do not see them. Sooner or later these little air travelers settle down. If they happen to land on a slice of bread or something else that they like to feed upon, they sprout a few tiny threads. Soon they add a few blurry colors and grow big enough to be seen.