You have already made yor own battery from two pieces of an orange.
Left: Hearing aids are powered by tiny "batteries" which are much smaller than a Pfennig coin.
Middle: Such a zinc air "battery" contains zinc powder in an alkaline solution called potassium hydroxide. During power supply zinc reacts with air entering the battery.
* Ask for a consumed zinc air battery, place it into a blister packing.
* Add water with a drop of an indicator for alkaline solution.
Observation right: The colour change indicates the presence of the alkaline solution.