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Microbes are living things too small to see without a microscope. They are the oldest life on Earth: fossils of microbe communities in Australia are about 3.5 billion years old.

Bacteria are single cells with no nucleus. They can be shaped like balls, rods or spirals, and most are less than five thousandths of a millimetre wide.

Viruses are much smaller: most are up to 100 times smaller than a bacterium. A virus is not a cell. It can only make copies of itself inside the cells of another living thing.

Experiment: Does yeast like it warm?

1. Plan a fair test: what will you change (water temperature), what will you measure (size of the balloon) and what will you keep the same?
2. Put exactly 1 teaspoon of yeast and 2 teaspoons of sugar in each bottle.
3. Add 100 mL of cold, room-temperature or warm water to each bottle. Measure and record each temperature.
4. Quickly stretch a balloon over each bottle. Start the timer.
5. Every 10 minutes, measure round the widest part of each balloon with string and a ruler. Record the results and draw a bar chart.

Time (min)	Cold: balloon size (cm)	Room temp: balloon size (cm)	Warm: balloon size (cm)
0	Predict which balloon will be biggest after 30 minutes and why.
10	Predict which balloon will be biggest after 30 minutes and why.
20	Predict which balloon will be biggest after 30 minutes and why.
30	Predict which balloon will be biggest after 30 minutes and why.

Questions

1. Write one difference between a bacterium and a virus.

2. Name two foods that are made with the help of microbes.

3. In your fair test, which balloon grew most? Explain why.

4. Why should antibiotics not be used for an illness caused by a virus?

Sources for every fact on this sheet: echoesof.science/school/microbes/sources/

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