A lemon. It’s a citrus fruit whose juice is acidic and can be used to conduct electricity for motors. A car or other purchase that has problems is known as a lemon and can be expensive to repair. Lemons have a tart flavor but do little harm (unless you get it in a cut, then it hurts like the dickens). The old saying goes, “When life gives you lemons, make lemonade” (I know, it’s spelled aid not ade but it sounds the same and wouldn’t make any sense the other way. It all works out nicely when the riddle is told instead of read.)