When Shrek hears that Fiona is pregnant, Puss tries to give him The Talk.
Shrek: How did this happen?
Puss: Allow me to explain. You see, when a man has certain feelings for a woman, a powerful urge sweeps over him.
Shrek: I know how it happened. I just can't believe it.
Donkey: (to Puss) How does it happen?