Follow these steps:
1. Place your can opener in a jar of white distilled vinegar and leave it to soak for a few minutes.
2. Remove the can opener over the sink, and use an old toothbrush to scrub away at the remaining dirt or rust. Dip your toothbrush in the vinegar to clean it.
3. Give it a rinse with dishwashing detergent and scrub again with the toothbrush.
4. Dry it carefully with a clean towel, paying attention to the gears.
Now it's as good as new!