In the Yeast and Sugar video, I added different kinds of sugar to bottles with yeast and warm water. One of the bottles was a control. What should have been in that bottle?

1. Just water

   No. With just water, you are removing two variables, the yeast and the sugar. You only want to remove the independent variable.
   

2. Water and yeast

   Yes! A control should be exactly like the others, but without the independent variable (the variable you are changing in the experiment.) In this case, the variable you are changing is the kind of sugar, so the control should have everything except for the sugar.

3. Water and sugar

   No. The yeast is not the independent variable, so leaving it out would not be correct.

4. Water and salt

   No. Adding salt would be adding a new variable, which is not correct.

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I wanted to test a new fertilizer, to find the best concentration for my garden. I divided my garden into four sections and put a different amount of fertilizer in each section.

My test results showed that using 10 grams of fertilizer per gallon made the plants grow faster and bigger. To follow proper scientific guidelines, what should I do next?

1. Apply 10 grams of fertilizer per gallon to all the plants in my garden.

   No. While that might make my garden grow well, it would not provide more evidence that this was the best mixture of fertilizer

2. Do the same experiment over again.

   Yes! Repetition is an important part of the scientific process. If my hypothesis is correct, I should get the same results every time I repeat the experiment.

3. Do the same experiment, but use a different fertilizer.

   No. Using a different fertilizer would be testing a different variable. I wanted to find the best concentration of the original fertilizer, so testing a different fertilizer would not help with that.

4. Publish my results, so that other scientists could replicate my experiment.

   No. Replication is an important step, but I should repeat my experiment several times to be sure that I get consistent results before I ask other scientists to try replicating it.

Click to see which state standards this question tests, and which of my videos, experiments, and other resources support that topic.