What makes a nutrient solution go rotten after a few days in a flask?

A nutrient solution can go rotten after a few days due to microbial growth, particularly bacteria and fungi, which thrive in nutrient-rich environments. Poor aeration, insufficient sterilization, and exposure to contaminants can accelerate this process. The breakdown of organic materials in the solution can also lead to the production of unpleasant odors and a change in color, indicating spoilage. Regular monitoring and maintaining sterile conditions can help mitigate these issues.