Why is the zero factorial one i.e ($0!=1$)? 
Possible Duplicate:
Prove $0! = 1$ from first principles
Why does 0! = 1? 

I was wondering why, 

$0! = 1$

Can anyone please help me understand it.
Thanks.

Answer 1: The "empty product" is (in general) taken to be 1, so that formulae are consistent without having to look over your shoulder. Take logs and it is equivalent to the empty sum being zero.
Answer 2: $(n-1)! = \frac {n!} n$ applied with $n=1$
Answer 3: Convention - for the reasons above, it works.
