Mohs Hardness Scale
Mohs Hardness Scale is a ranking of ten minerals from softest to hardest based on which ones can scratch the others.
Think of it this way
A kitchen drawer holds tools made of different materials. A steel knife can scratch a wooden spoon, but the spoon cannot scratch the knife back. A soft rubber spatula gets marked by both. This is how the Mohs scale works: harder things scratch softer things, but not the other way around.
Explaining mohs hardness scale by grade level
Some rocks are soft and some are hard. You can test this by trying to scratch one rock with another. If a rock leaves a mark, it is harder. Talc is so soft you can scratch it with your fingernail. A diamond is so hard that nothing else can scratch it.