ELI5: Explain Like I'm 5
Examples: how to be happier, mathematical finance, water, salmonella, expected value

Hardness comparison

Hardness is a measure of how resistant a material is to scratching or being worn away. To compare hardness, we use a scale. The hardest material is given a score of 10 and the softest is given a score of 1. Materials with higher numbers are harder than materials with lower numbers.