Because they don't know the words.
or
Do you believe in humans?
or
Take away the g
or
Because of the bark
or
They both have a hole in one!
or
and the other one says, "That's what I was going to say."
or
Porkchop
or
"Nobody said I needed doubles on that!"
or
Tell him the key signature has 8 sharps.
or
They're both extinct.
or