Q:What's the difference between a sweet potato and a yam?
A: True yams are pretty much never seen in a typical Western supermarket, though you may find them hanging out underneath the produce display if you shop in an area with a large South African or West Indian population. What we sometimes refer to as "yams" are technically all sweet potatoes, not yams. That said, this whole debate is like the one about barbecue, or whether a tomato is a fruit or vegetable. Words have fluid definitions that can vary based on the context in which you use them. If you want to call your garnet sweet potatoes "yams" because that's what your Southern mother-in-law knows them as, then by all means, keep calling' em yams, and feel free to yell down your pedantic relatives.