1.The metal strips and potato make a simple battery that creates electricity to operate to operate the clock. Each potato works as a device called a electrochemical cell. It converts the chemical energy stored in the metal strips intoelectrucal energy.Two potatoes are needed to make electricity strong enough to run the clock.
2.A cell works because of the chemical properties of the metals inside (in this case the copper and zinc).The different properties cause tiny particles charged with electricity . (called ions) to move between the the two strips of metal. This flow is an electric current. The potato contains the particles that allow the current to flow,but it stops the metals touching.Electric current also flows along the wires between the zinc and copper strips and the clock. This current makes the clock work.
You may experiment with different liquids like salt water,fruit juices.Or fruit like lemon, orange,tomato etc.The fun is unlimited.