A decaying peach over six days, with each frame about 12 hours apart. The fruit shrivels and becomes covered with mould.

Decomposing fallen nurse log in a forest

Decomposition, or rotting, is what happens to livng things when they die. It also happens to organic material of any kind, such as foodstuffs. The reason it happens is that organic molecules are stores of building blocks and energy. Whatever breaks down organic material uses the energy and building blocks to reproduce itself.