≤ 2.5 - Fingernail

≤ 2.5 - Fingernail

≤ 2.5 - Fingernail--is scratched by a fingernail

  ≤ 3.5 - Penny

≤ 3.5 - Penny

≤ 3.5 - Penny--is scratched by a penny

 ≤ 5 - Nail

≤ 5 - Nail

≤ 5 - Nail--is scratched by a nail

  ≥ 5.5 - Glass

≥ 5.5 - Glass

≥ 5.5 - Glass--scratches glass