Louis Pasteur, after testing with a microscope, found that almost everything had microorganisms in them. He also found that there was bacteria in human blood which he referred to as 'Germs'. He thought that these germs lead to diseases spreading. So he did an experiment.First he boiled a broth to kill all of the germs in it.He used a specific flask to keep any air from entering.
He waited a few days. Nothing grew.
He waited a few more days after that. Still nothing.
He redid the experiment but he did something different after the first few days.
This time he broke the end of the flask to expose the air to the broth.