Inside a PC's optical drive. It worked much better without the M&M's someone decided to feed it, rattling about in there...

Don't ever open one of these up when they're powered up. The LASER emits light that is invisible to the human eye, so you wouldn't be aware of it until too late. Then you'd become aware of the permanent damage and losses to your eyesight.

Most common faults with such drives - dust on the lens. There are special Compact Discs with teeny brushes on them available. These can be used as the first attempt to clean the lens, and don't involve any dissection of the equipment. Second attempt would be using the likes of a cotton bud with a couple of drops of IPA (isopropyl alcohol - a solvent cleaner, often used in photographic lens cleaners), and a very gentle wiping across the lens. Always ensure it has evaporated from where it's been applied, prior to switching on - it's highly flammable, and I have witnessed a fireball caused by not heeding such advice, by one numpty. He almost lost his eyebrows that day! And no, it wasn't me.