Write down what you think might happen (your hypothesis). Get an empty soda bottle. Partially fill it with water. Accurately mark the water level on the side of the bottle with tape, permanent marker, etc. Put in freezer overnight. Observe and make note of any changes to the level of the bottles contents. Compare your data with your hypothesis. If you can repeat the experiment and obtain the same results, as well as have other scientists do the same, then your hypothesis can become a fact of science.