Physics
It is more difficult to walk on a sandy road than on a concrete road because

the sand is grainy but concrete is smooth.
sand is soft and concreter is hard.
the friction between sand and feet is less than that between concrete and feet.
the friction between sand and feet is more than that between concrete and feet.

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Physics
The sky appears blue because

In white light, the blue component dominates
It is actually blue
The atmosphere scatters blue colour more than the others
All colours interfere to produce blue

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