why do egg shells stop slugs from going there, is it just the sharpness or is there a chemical reason?

Question: why do egg shells stop slugs from going there, is it just the sharpness or is there a chemical reason?
- Keywords:
  chemical,
  egg,
  shell,
  slug
Asked by calvis to Mike, Pip, Tianfu, Tim, Tom on 29 Jun 2012.
- Tom Lister answered on 29 Jun 2012:
  
  I can’t get this to work! I guess it is the sharpness though.
- Tim Stephens answered on 29 Jun 2012:
  
  I don’t know if it works. My tomatoes still get eaten

Question: why do egg shells stop slugs from going there, is it just the sharpness or is there a chemical reason?