What's a possible hypothesis for the effect water has on plants?How would the control group be teated differently from the experimental group?
Since a hypothesis is a prediction that can be accurately measured for confirmation or not, possible hypothesis statements could be "Geraniums need one-half cup of water every day to grow" or "Geraniums need one-half cup of water once every week to grow" or "Geraniums need one-half cup of water once every month to grow."
If you are attempting to measure the effect of water on the growth of plants, varying the amount of water given different plants would probably be the easiest variable to use for your experiments. Alternatively, you could experiment with different temperatures of water or see if the time of day when the water is applied has any effect on the plants. The key, as with any experimental data, would be to have your control group, your group(s) of plants that receive varied treatment from the control group, and careful recording of all the procedures used and results observed.