They can't unless they are between the age of 10-12 months because at that age they are maturing and almost a full grown dog. But if they are younger than that it is very unlikley that it would get pregnant.
How puppies get pregnant. Hmm. Well, puppies are a product of their mommadog getting pregnant. As people have said before me, puppies are baby dogs. Just like a human, they have to develop their insides i.e. hormones and reproductive systems to make other puppies. Same with boy pups. When they reach a certain age, as LOL13 mentioned above, they put out a scent that attracts boy dogs. The boy dog and girl dog like each other, just like humans, and they engage in sharing bodily fluids. The girl dog is pregnant and she will have puppies. I Strongly Suggest You Do NOT encourage this process!!!!! Let them be puppies just like you should be "kiddies" as long as you can. Do the opposite for your puppies and get them fixed so they don't get pregnant and have more puppies! Have a nice day!