Roll the lemon on a table gently to release the juices inside without breaking the skin.
Push the copper wire in the lemon through the skin about ½ inch. If you don’t have copper wire, make a slit in the skin using a knife and insert a copper penny instead.
Insert the nail or a steel paper clip into the lemon as close to the copper wire or penny as possible without touching.
Place your tongue on the penny and the nail or paperclip next to it, or on the tips of the copper wire and the nail/paperclip at the same time.
You should feel a slight tingling on your tongue that means the lemon battery is working.
Measure voltage if you have a multimeter or volt meter available. Attach the meter's positive and negative wire end clips to the copper wire or penny, and the galvanized nail or paperclip. Then check the voltage readout to see if the lemon battery has power.