Counting bacterial colonies on an agar plate tells you how many germs were in a water sample. In this experiment, you swab water from three different sources onto agar dishes before and after leaving each bottle in direct sunlight for two days. After five days of growth, you measure the colonies. Smaller groups on the sun-exposed plates mean the water had fewer bacteria — showing how much ultraviolet rays can reduce bacterial growth.

The sun produces ultraviolet light that can kill germs in water. Those UV rays damage bacteria cells directly. When water bottles sit in sunlight for two days, the colonies in sun-exposed water are much smaller than in water kept in the dark.

Ultraviolet rays from the sun can damage bacteria cells — but does that effect work on real water sources? This project tests exactly that. You collect water from three sources: a tap, a drain, and a lake. Before and after leaving each transparent bottle in sunlight for two days, you swab samples onto agar dishes and let colonies grow for five days. The results show how much smaller the colonies are in sun-exposed water compared to the originals, revealing how much the sunlight reduced bacteria growth.