Why is it difficult to walk on sand?

Why is it difficult to walk on solid ice?

Why is it difficult to aim and hit targets under water, such as fish?

Why is it difficult to climb up a greasy pole ?

Why is it difficult to hold a school bag having strap made of thin and strong string?

"It is difficult to run fast on sand ". Why?

Why is it difficult to prepare pure amines by ammonolysis of alkyl halides?

Why is it difficult to differentiate between red and green colour objects in dark or in night ?

Explain, why is it difficult for a fireman to hold a hose, which ejects large amount of water at a high velocity?

Why has it been difficult to study the chemistry of radon?

Why has it been difficult to study the chemistry of radon?