0_Apprentice
My brother just broke his arm, I wonder how much time he will have a cast. 
1_Wizard
A bone fracture is a medical issue when there is damage to the bone continuity. 
0_Apprentice
How do bone fractures heal ?
1_Wizard
The blood then tends to coagulate to create a blood clot that is between the broke fragments.
0_Apprentice
But what happens to the blood clot if the bones move ?
1_Wizard
Then  ithin a few days the blood vessels expand into a jelly like material of a blood clot.
0_Apprentice
ok, so why people do have to wear a cast when they broke a bone ?
1_Wizard
Then the new blood vessels carry Phagocytes to the injured area which kick out all of the non-viable material.