In your experiment, the control is the condition against which all other conditions are compared. In the context of your experiment, the control would be the ball with no stripes that you left in a room at a temperature of 20 degrees Celsius. This is the baseline condition, allowing you to assess how the other two balls (one at 0 degrees Celsius and the other at 40 degrees Celsius) perform relative to this standard temperature.
When conducting experiments, controls are crucial for ensuring that any differences in the results can be attributed to the variable being tested—in this case, the temperature of the balls—rather than other factors. By using the ball at room temperature as your control, you can then observe how the bounce height of the balls changes based on their different temperatures and compare those results.