A slug's blood is green.

The story behind the answer

True—many slugs have green-tinted blood (hemolymph). Unlike mammals, which use iron-based hemoglobin (red), slugs and other mollusks rely on copper-based hemocyanin to carry oxygen. In some slug species, additional pigments such as biliverdin mix with hemocyanin, giving the blood a green appearance.

Fun context: blood color depends on the molecule that binds oxygen. Hemoglobin makes ours red, hemocyanin makes many mollusks’ blood blue to green, and some worms have greenish chlorocruorin. Memory tip: think “garden slug, garden green”—the slimy garden dweller carries green in its veins.