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The materials required for the science fair project experiment:
- 2 agar petri dishes
- 2 disinfected swabs
- 1 bottle of disinfected water
- tap water
- 1 water dispenser and water cooler
- 1 marker pen


1. For this science fair project, the independent variable is the source of the water – ie: tap water or water cooler water. The dependent variable is the size of the bacteria growth in the petri dishes. This is determined by measuring the size of the bacteria growth using a ruler. The constants (control variables) are room temperature, the amount of sunlight and the ingredients in the petri dish agar.

2.  Two petri dishes  are prepared and  stored in a refrigerator. The petri dishes are brought to room temperature before the start of the science fair project by taking them out of the refrigerator and allowing them to settle at  room temperature. The petri dishes are labeled A and B.

3. The sterilized swab is first dampened with sterilized water.  The first  swab is soaked by placing it under the running tap water and then rubbed across the agar in the petri dish labeled A. The second swab is soaked in water from the water cooler and rubbed across the agar in petri dish B.

4. The cover is closed and the 2 petri dishes are kept in a cool dark place for the bacteria to grow.

5. The size of the bacteria colony is measured for the next 5 days and recorded.

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Complexity level:
Project cost ($):
Time required:
1 day to prepare, 5 days for science fair project
Material availability:
Easily found
Safety concerns:

The bacteria should be destroyed before being disposed of. Pour some bleach into the petri dish to kill all the bacteria before disposal.