This lesson should lead to a discussion of the role of microorganisms in food spoilage. As the microorganisms use the cucumber as a food source, they decompose it. The cell walls break down and the cucumber becomes mushy. As the microorganisms multiply, they scatter the light shining through the cup and the water becomes cloudy. Moreover, these ideas above could lead to a discussion of microorganisms in general.
Some microorganisms cause diseases.
That is why it is bad to eat food that has been left out for too long. It can make us feel sick. However, other bacteria are essential to maintaining health. For example, there is a certain type found in our guts which works to maintain the proper type of environment. It is the same bacteria which makes yogurt taste sour while at the same time making yogurt so nutritious.