Why are the core and mantle still so hot? which layers of the earth are solid vs. liquid and why?

The mantle and core of the earth are still hot because of the pressure of the overlying earth layers. The deeper the layer the hotter it is due to higher pressure, in lieu of the fact that temperature and directly proportional. In addition, some of the heat from the time the earth was formed is still trapped inside the earth and nuclear decay of radioactive elements in the regions ensure the earth does not cool.

The mantle and outer core of the earth are liquid while the inner core is solid. The former  layers are liquid because the temperatures in these regions have surpassed the melting point of the elements that make up the layers. However, the inner core remains solid due to the enormous pressure in the region. The pressure is so high it prevents the element in the region from melting.  

stress fractures occur largely in the feet and legs and are common in sports that require repetitive impact, primarily running/jumping sports such as gymnastics or track and field.