Laurie Kreutz, age 10, of Spokane, Washington, for her question:

How can a wet haystack set itself on fire?

This is possible because certain bacteria thrive in grassy meadows. They are there waiting in the hay when it is mown and stacked. These particular bacteria like moisture. If the haystack is wet, they begin to feed and multiply at an enormous rate. Their life activities give off heat. In order for a fire to get started, it needs oxygen and enough heat to ignite its fuel. Hay does not need as much heat as coal or logs to start burning. And those living, thriving bacteria can provide enough to start it smouldering. The smouldering hay gives off enough heat to set itself on fire.

This sort of thing is called spontaneous combustion, which means fire that starts by itself. Actually the haystack was burned by small arsonists called bacteria. Spontaneous combustion also can occur when sloppy people leave paint rags piled in garages or stuffy closets. In this case, the heat is provided by chemicals used in the painting job.