Before Pasteur's experiments using beef broth and flasks with S-shaped tubing, most people thought that new cells formed by

spontaneous generation

Explanation:

Spontaneous generation is the idea that living organisms arise from nonliving matter. Louis Pasteur was a French chemist who designed a series of experiments to determine whether a sterile nutrient broth could spontaneously generate microorganisms. He added nutrient broth to two flasks, curved their necks into S shapes, and finally boiled the broth to eliminate any potential microorganism. Pasteur did not observe any microorganisms in the flasks with the S-shaped necks, thereby showing that microorganisms present in the air were responsible for contaminating nonliving material and thus rejecting the idea of spontaneous generation.

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