Here's why:

* Spontaneous generation is the discredited theory that living organisms could arise from non-living matter.

* Experiments on spontaneous generation aimed to test this theory by observing if life could emerge from non-living matter in controlled conditions.

* The key variable is whether a source of pre-existing life (like microorganisms) is introduced into the experiment or excluded.

Examples:

* Pasteur's famous swan-neck flask experiment:

* Controlled variable: The presence or absence of microorganisms in the broth. The broth in the flask with a swan-neck was exposed to air but not to any existing microbes. The broth in the open flask was exposed to both air and microbes.

* Result: Only the open flask showed signs of life, proving that life does not spontaneously generate but arises from pre-existing life.

In essence, the controlled variable in spontaneous generation experiments is the variable that determines whether or not a source of pre-existing life is allowed to contribute to the generation of new life.