Theory:

The hardness of a material shows how solid it is in comparison to other materials.
 
Let us look at this through the activity.

Step 1: Hold a chalk piece in one hand.
 
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Step 2: On the other hand, take a small stone.
 
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Step 3: Use the strength of your hand and try to crack them into small pieces.
  
Are you able to break all materials with your hands?
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Breaking a chalk piece is considerably easier than breaking a stone as the stone is much stronger than chalk. Stronger materials are more difficult to break.
 
As a result, we can use this type of stronger material for many purposes, including defence. For example, we use stones and iron rods to construct weather-proof houses.
 
Soft materials are those that can be quickly compressed or scratched, while hard materials are those that are difficult to compress. For example, cotton or sponge is soft, while the iron is a hard material.
 
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A sponge is a soft material