Airplane conversation

A stranger was seated next to a woman on the airplane when the
stranger turned to her and said, 'Let's talk. I've heard that flights
go quicker if you strike up a conversation with your fellow
passenger.
The woman, who had just opened her book, closed it slowly and
said to the stranger, 'What would you like to talk about?'
Oh, I don't know', said the stranger. 'How about nuclear power?'
'OK,' she said. 'That could be an interesting topic. But let me ask
you a question first. A horse, a cow, and a deer all eat grass, the
same stuff.

Yet a deer excretes little pellets,

while a cow turns out a flat
patty,

and a horse produces clumps of dried grass.
Why do you suppose that is?
The stranger thinks about it and says, 'Hmmm, I have no idea,'
To which the woman replies, 'Do you really feel qualified to
discuss nuclear power when you don't know shit?'