Few elementary school science supply closets have a hot plate but this allows teachers to enlist the aid of the cafeteria staff who will supervise the boiling process on their stove. The liquid can be boiled away directly or using a double boiler method. The former results in 'charred remains' that look like sugar burned in the oven when the cherry pie spills over. The double boiler process results in a deposit of cola syrup. I place a diet drink into the water and it sinks.
Students are stunned.
In many parts of the world they are still using iron-based containers. A magnet determines this rather quickly. Then show them the oddity. What students learn about variables will be important in other curriculum areas. Learning how to test for the role of each will immerse them in thought although the exercise may be mentally draining.