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Pasteur produced a nutrient broth and placed equal amounts of it in two long necked flasks. One flask had a straight neck (control) and the other had a swan shaped bent neck (being tested). He boiled the broth in both the flasks to kill any living organism that might be surviving on it. The two flasks were then left in the open air of the room to cool down. After several days passed, Pasteur noticed that the broth in the straight necked flask had changed its color but the swan necked flask didn't. From this observation he was able to conclude that the straight necked flask had let the organisms enter the flask and contaminate the broth. The swan necked flask blocked the entry of the organisms from the open air and thus remained unaffected.
