I recently bought a 2nd hand iMac.
It’s a few years old but thought it would be great for the kids to learn on.
The Keyboard was in a filthy state on the surface, so I decided to do a full clean (which included removing the individual keys). What I found underneath was evidence of the previous user frequently enjoying their lunch while using the computer. (looks like crumbs…plus there’s some hair, dead skin particles and who knows what else.)

If you’re thinking of cleaning your own keyboard, unplug it from the computer and grab a damp cloth (no detergrent although some rubbing alcohol is useful) a can of compressed air and a digital camera (to take a photo of your keyboard before you remove any keys – just in case you can’t remember where to put them.)