Batteries
Batteries generate electricity via the chemical reactions that go on inside them. The components that cause these reactions deteriorate over time, which is why batteries have an expiration date. Once this date is passed, the batteries will not be as effective, and may not even work at all.
Thankfully there are no health implications to an expired battery, but it should be disposed of carefully and not with your normal household waste. You can buy voltmeters which will check how much life is left in a battery, to save you throwing away a perfectly good one, or to let you know when you need to buy spares.