Why do our legs swell?

Why do our leg muscles cramp when we run for a long period ?

Why dough rises swells, when it is treated with yeast ?

Why does your ankle swell up when it is sprained?

Why do birds swell their feathers in winter?

Why do our body cells require oxygen ?

Why does borax swell up on heating?

When we step on a thorn, we withdraw our legs suddenly, if we are taking this thorn out, we will not withdraw our legs. Comment on these two statements.