how the following printf statement works

From: aditya (adityavasishth_at_gmail.com)
Date: 10/29/04 

Date: 29 Oct 2004 01:59:36 -0700

hi,

Can anybody please tell me that how the following printf(...)

statement works-
 
 main(){
   int d=9;
   printf("%d",printf("%d"));
   return 0;
}

The output is 91.

Thanks,

Aditya.