Subject: How does planetismal theory of earth explain molten core?

I cant find this topic discussed anywhere on the net, but it has occured to me 
(after visitig the planetarium a few weeks ago), where they expoundedthe 
planetismal theory of earths formation - that this theory seems to be at odd's 
with the fact earth has a molten core. ie youve got all these small, to medium 
size 'planetismals' cooling in space, then "sticking to one another due to 
gravity", they get bigger and bigger, and eventually become the size of earth, 
but how in the midst of that does the earths core become molten???

Re: How does planetismal theory of earth explain molten core?