how make concludes that target is out of date

[sharan basappa]

I have following make rules in my file

one : a.c
       echo "hello"

two : c.c
       echo "hello"


a.c exists and c.c does not exist ..

My question is

a) How make takes up a rule and decides that target is
out of date and hence command needs to be executed.

b) If the target is not a file but prereq is and this preq file exists then 
what happens.
Rule one above a.c exists but make exits with message make: `one' is up to 
date.

c) If target exists but prereq does not. If you refer to second rule above 
then
make does not execute the command echo "hello" and exits with message
make: *** No rule to make target `c.c', needed by `two'.  Stop.

I would be glad if someone could point me to right documentation to 
understand better

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