Here is an excellent demonstration from an excellent teacher (although don’t forget to involve the students).
This project is very easy to do! What you need are an aluminium can, a thin copper plate, two spacers of rubber, a motor of 2-3 volts e.g. CD ROM motor, 1 litre of water with 2-3 tablespoons of salt.
The duration of the time of rotation of the motor is over 7 minutes. When the battery runs out you can reof its charge shall be filled again with water and salt. To secure the success of this experiment, it is necessary to have a motor with a low power consumption 15-20 mA max. and a voltage of 2-3 volts. It is not recommended to use these motors in toys.