ELI5: When opening a can with a can opener, what keeps metal particles from falling into the food from where you're cutting?

The amount of "dust" that is produced is quite small, and is mostly from scratching off the surfaces - we're talking about a few atoms being scrapped loose where the cutting edge scrapes into the can.

Metal deforms - like aluminum foil crumpling when you press on it - more than it crumbles. You don't really get crumbs of metal (unless a large, obvious piece breaks off) - you get a tiny sprinkle of a few atoms that get knocked off.

You can test this for yourself with a new penny and a butter knife. Scratch up a penny with a knife, and then rub the scratched-up penny on your palm for a minute. Now smell your palm - that metallic smell is the copper plating you rubbed off of the zinc coin. But you'll notice that you don't see any visible dust or flakes - because it's such a small amount of metal.

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