why if we pad any signal with enough zeros we can get the same result if we pad any signal with enough zeros we can get the same result as linear 
convolution whilst using the fft function we compute circular convolution. 
Why?
Hint:  try computing the linear convolution of $~[1\quad 1\quad  1]~$ and 
$~[0\quad 1 \quad 1]~$ by hand and the circular convolution of the zero-padded
versions
$~[1\quad 1\quad  1 \quad 0\quad 0\quad 0]~$ and $~[0\quad 1 \quad 1\quad 0\quad 0\quad 0]~$ also by hand (or via FFTs if you are desperate enough).