What's commonly added to rubber compounds to make a tire stick to pavement in wet conditions?

• silica
  If you plan to ride in the rain, spending a little extra for a silica-enhanced tire is likely worthwhile.
• Gore-Tex
  If you plan to ride in the rain, spending a little extra for a silica-enhanced tire is likely worthwhile.
• safflower oil
  If you plan to ride in the rain, spending a little extra for a silica-enhanced tire is likely worthwhile.