Comparing function return value in C

I was making a program and I noticed that doing
if ((j = function()) == 20)
and
if (function() == 20)
behaved differently. Why is this? What's the difference between one and the other? The first one (the one that has a variable with the function return value) behaved as I thought it would, which is comparing the function return value with the constant. I don't know what the other one does. Thanks.

>>5
And this is why mutable state is considered harmful.