One of JJ's friends recently hosted a science themed birthday party for his 7yo, complete with science experiments.
First up was lemon batteries. They had fun poking various pieces of metal into the lemons and then tried to light a small LED from it.
Unfortunately the lemons didn' put out enough power to light the LEDs on thi occasion (it had apparently worked when JJ's friend was testing out suitable experiments for the party). They moved on to the next thing while JJ used a metal coat hanger to skewer some of the lemons into a parallel to see what kind of charge he could get and apparently we'd need quite a lot of lemons to get one small LED to turn on.