Slugs won’t cross copper because it imparts a slight electric charge when they touch it. Dark-colored, older pennies, which are mostly copper, work the best. The trick is to get the penny into direct contact with the slug’s skin. If it makes physical contact, the slug will try to crawl around or over but won’t be able to because of its slime trail and gravity. The slime trail provides a path for electric current and causes paralysis while gravity pulls them down.
Here’s the instructions for a neat “penny ball” you can make to deter your slugs while adding some interesting art to your garden!