Others have covered the copper angle. But it doesn't have to be a potato. A bit of paper towel moistened with salty water or saliva would also work, all you nee

Others have covered the copper angle. But it doesn't have to be a potato. A bit of paper towel moistened with salty water or saliva would also work, all you need is for current to flow initially (that's the purpose of the salt), and for something to carry the copper ions (the water).