Why do athletes like sprinters have a higher proportion of white muscle fibre?

Athletes who participate in events like marathon or swimming over long distance have higher proportion of red fibres.

Why do stable nuclei never have more protons than neutrons?

Why do the veins have valves?

Why a red muscle can work for prolonged period whereas white muscle fiber suffers from fatigue after a shorter work?

Why do the blades of a pair of scissors have edges ?

Why do animals in cold regions have a layer of fat below their skin.

Why do the load carrying heavy vehicles have large number of wheels ?

Why do racer cars and birds have typical shape ?

Why do stable nuclei have more neutrons than protons?

Why do school bags have broad shoulder traps ?