Where does rot come from? Around 1668 the Italian physician Francesco Redi tested whether living things can appear by themselves out of rotting material. Jars covered so flies could not lay eggs stayed free of maggots, so young flies come from flies' eggs, not from the rot. His test had a clear difference between the jars, and it gave a clear answer about where young flies come from. Rotting material did not make them appear by themselves, and the result with the covered jars is what pointed to flies' eggs.

The same question came back for the tiniest living things. In 1861 Louis Pasteur heated broth in a flask with a long S-shaped neck, called a swan neck. Air could get in, but dust was trapped in the bend, and the broth stayed clear, but when he broke the neck, the broth spoiled. Air got in both times, but after the neck was broken the dust could go straight in. That is the logic of a fair test: change one thing, here the neck of the flask, and see what happens to the broth.

Experiments on rot can be slow, and some of them are quiet and simple, because the thing being watched is only a block of wood in the ground. To study rot, scientists at Harvard Forest bury blocks of wood in soil and see how long they take to rot and disappear. Here the measuring stick is a block of wood, and the clock is how long the block lasts in the ground. A block that takes longer to disappear has rotted more slowly, so the time it lasts is the measurement. Slow tests like this one trade speed for a plain answer: how long the wood lasts.

Plant cell walls are built of three fibers. Hemicellulose is softest, cellulose is sturdier and lignin is toughest. Cellulose is the middle fiber, sturdier than hemicellulose but not as tough as lignin. It matters for one wood-eating animal in particular. Termites eat wood but cannot digest cellulose alone. Bacteria, fungi and protozoa living in their guts break the cellulose down so the termites can absorb the nutrients. Even an animal that eats wood needs help from the microbes inside it. The termite's helpers live in its gut. So the termite and its gut microbes work as a pair.

A fallen tree is worked on by a series of different living things. After a tree falls, the first workers are usually fungi that specialize in wood, using their hyphae. Then bacteria and beetle larvae get inside, and the holes they make let in water, which helps fungi and bacteria thrive. Some mushrooms grow on dead logs and stumps, and they are decomposer fungi. The first workers open the way for the ones that come next. Rot in a log is a team job, with different workers at different stages. The water gets in through the holes.

Soil is busy at a scale that is hard to picture. There are more microorganisms in a teaspoon of topsoil than there are people on Earth. Count every person on the planet, and then picture that crowd of tiny living things fitting in one small spoonful of dirt. Earthworms add to the work, since they can eat up to their own weight in a day, which keeps organic material out of landfills. These two facts show how much of the work in soil is done by living things that are easy to overlook.

A home compost pile puts numbers to the job. Microbes in compost work well at carbon-to-nitrogen ratios from 25:1 to 40:1, and a mix of 30 parts carbon to 1 part nitrogen is considered ideal. For a home pile, EPA advises at least two to three times as much dry leaves and other browns as food scraps by volume, and a cover of four to eight inches of browns over scraps. Cheese and greasy food should not go in a home pile, because they can attract animals. Compost also helps soil hold water, and it feeds plants by letting nutrients out slowly. Those numbers are for a home pile, and they show why a mix of browns and greens matters.

Three facts from the kitchen show that food keeps changing. Mould spores are microscopic, from 2 to 100 microns across, and many are so small that they float through the air. Spores turn up both indoors and outdoors, yet growth happens only where there is moisture. Cold slows mould but does not stop it. Mould can still grow on forgotten leftovers in a fridge. Fruits are sorted into two types. Climacteric fruits, such as apples, bananas, peaches and avocados, can keep ripening after picking. Non-climacteric fruits, such as grapes, cherries and strawberries, cannot.

In chemistry and genes, fungi are closer to animals than other organisms are. Yeasts, which are fungi, make the bubbles in bread, so the bubbles in bread and the rot on a log both come from the same kind of living thing. Scientists estimate there may be about 2.5 million species of fungi worldwide, as of November 2026. Only around 155,000 have been formally named, so most are still unknown. That is about 6 in every 100 named, so the named ones are a small slice of the whole group. As of November 2026, most of the fungal world still has no name.

Words to know

• spores: tiny mould seeds
• hyphae: fungus threads
• microscopic: very tiny
• organic: once living