Measure the temperature of the water by placing the metallic tip of the thermometer in the water.
Place the container of water in the freezer if the water temperature was greater than 39.2 degrees Fahrenheit (4 degrees Celsius). If it was lower than that, you can leave it out at room temperature, or microwave it for a short time, to raise it.
Check the water's temperature periodically. Remove the container from the freezer or microwave when it reaches around 39.2 degrees Fahrenheit. At this point it will be at the maximum density for pure water.
Pour approximately 4 tbsp of salt into a cup of water. If you need to increase the density of a greater volume of water, use proportionally more salt.
Stir thoroughly until the salt completely dissolves in the water.
Pour the salt water through a paper towel into another container. This will remove any undissolved salt particles and keep the volume the same as before. Since you have added mass without increasing volume, the density of the salt water is greater than the pure water with which you started.