If you're trying to test for the cooling curve of water, then before you start your experiment you're going to have to declare the controlled variables.
What does declaring the controlled variables mean? It simply means you have to list all the factors of the experiment that you have control over, and exactly what influence they will have on the results.
For example, if they are going to alter some of the data you collect due to a systematic error, then now would be a good time to declare that.
Here's a good example of a list of controlled variables taken from an experiment used to find the cooling curve of water:
Controlled Variables: The controlled variables used in the experiment are:
A 400ml beaker with the systematic error of ± 50ml,
200ml tap water,
A Bunsen burner,
A stopwatch with the systematic error of ± 1 second
A thermometer with the systematic error of ± 0.5 °C.
