Why do some kanji change sound?

[QuakerBits]

So, not sure if this is in the right area of the form... anyways.

So here's 2 examples:
→手紙
→灰皿

I'm confused how the か in 紙 becomes が. Same with the さ in 皿 becoming a ざ sound、and so on for other kanji. I know this is probably to make it easier to pronounce, but I know of no rules of this to make it "understandable", or what it's even called.

Unless this is quick to explain, please do provide a link so I don't waste your time.. If this was posted already, I have no idea how to search it, so I'm veddy sorry.

[phreadom]

http://en.wikipedia.org/wiki/Rendaku

That should give you an introduction to it. I don't know enough to give any better explanation myself. :/

[Ongakuka]

The reason is that it is easier to say (or at least flow better.) I see Phreadom-san has posted a link so I guess that will cover everything else

[QuakerBits]

Thanks a lot guys(: And thanks for the fast response!!!!!