Lisa Ling goes undercover with animal activist Bill.

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To see what goes on at puppy mills, Lisa Ling brings a hidden camera and joins Bill on a tour of puppy mills in Pennsylvania. According to The Humane Society of the United States, there may be as many as 10,000 puppy mills operating across the United States.

Lisa and Bill also visit two pet stores. Many of the dogs born in puppy mills end up in pet stores or sold on the Internet. Lisa and Bill want to trace puppies' paperwork back to specific puppy mills. At the first store, they find a cockapoo puppy, and at the second, Labrador retriever puppies.

While the owner of a store they visit denies his dogs come from puppy mills, Bill says most at pet stores do.

"The fact is, what they're doing is not illegal," Lisa says. "But the objective I think here is to raise awareness. People go into the pet stores, and they see these cute little puppies not knowing where they come from and what they go through."