Any number of experiments can be designed that include an independent variable, dependent variable and constants. A variable is something that can change and can be measured. An independent variable is a variable that we change in an experiment. And the variable that is changed by the change in independent variable is the dependent variable. A constant is something that does not change its value during the experiment. Lets design as experiment including all three.
Let us try to figure out if sunlight is needed for plant growth. In this experiment we can use two pots each containing the same plant of about the same age and health and put one of them indoors and the other one outdoors. The presence (or absence) of sunlight is the independent variable over here. We will continue to water the plants at the same frequency and in the same quantity (say 100 ml per day or so). The new number of leaves and general state of plant health, after a week, will give us an idea of the need of sunlight. We can also term the number of new leaves as the dependent variable. Here, the type of plant and frequency of watering are the constants.
We can similarly design an experiment to study the effect of temperature on the time it takes to make tea. Here, temperature would be the independent variable, time taken for steeping will be the dependent variable and the process (amount of tea, water, heat source, etc.) will be the constant.
Hope this helps.