Answer
Through a curved flask
Explanation:
Before Pasteur's experiment, the common belief was that of spontaneous generation. That is, that living things could exist from non living things. Therefore, it was assumed that if you boiled a pot of broth and left it exposed, microorganisms grew in it from non-living air particles, not pre-existing microorganisms. Pasteur disproved this by creating a curved flask, that would allow air to enter the liquid, but was not big enough to allow microorganisms to enter the flask. Thus, all the microorganisms got stuck in the curve, and no organisms were found in the broth even with air exposure.
