Pour about 200 mLs of tapwater into a 250 mL beaker. Using one of the alligator clips, clip the copper strip to the rim of the beaker so that at least part of the copper strip is below the surface of the water. Across from the copper strip on the other side of the beaker, do the same with the zinc strip and the other alligator wire. (You can move the copper and zinc strips around, but don't let them touch.)
Your setup should now look like this: A copper strip clipped to the rim of a beaker of water on one side, and a zinc strip clipped to the same rim of the same beaker of water, just across from the copper. This leaves you with two free alligator clips. One is from the zinc; the other from the copper. Connect the two 'gators to the two leads from the multitester, and, presto, you should observe about one volt of electricity! Experiment by adding a drop or two of lemon juice to the mixture. Does the voltage change? Try other drops of mild acids such as orange juice, vinegar, etc.
What is happening here? The copper is acting as a positive terminal. The zinc is the negative. Free electrons are breaking away from the zinc and migrating across the water, forming ions. The ions being accepted by the copper, and a battery is born!
This experiment works well if you have a digital multitester. You can watch the acids you drop in cause the multitester to indicate a spike in the voltage. Have fun!