I must take a brief interlude from my usual grammar and style ramblings to discuss the dilemma of the American yam, specifically the yams that we see in U.S. grocery stores. Let's say you're ambling down a clean well-lighted aisle, and there you see, between bananas and okra, thick red yams and sweet potatoes. But alas, poor Yorick, all this time you thought they were the same: plain potatoes, in the morning, standing four inches in one hand. Sweet potatoes in bags. Yams at school. They were the same on the dotted line. But in your arms they were always different.
Yams are root vegetables, like sweet potatoes, but of the Dioscorea genus—closely related to lilies. While yams can be similar in size to sweet potatoes, they can grow thicker and longer—multiple feet longer! Yams are starchier, drier, and, based on an intensive Google image search, whiter (though they can also have that seductive stale orange hue). What's worse is that true yams are difficult to find in U.S. grocery stores, and when they are, they usually come from the Caribbean or some other exotic place.
The sweet potatoes we buy are, in fact, sweet potatoes. But what they never told you is that we have two major types of sweet potatoes: soft and firm. Unfortunately, for humor's sake, the soft variety did not come first. Firm sweet potatoes were first erected in America, and we can't have two different things with the same name, so we needed a new name for the soft type. Thus, through marketing skills and good charm, someone named the soft sweet potatoes yams. If you're in some hipster grocery store that has real yams (or an Asian market, as I learned a minute ago), you'll probably see yams beside yams beside sweet potatoes.
But if you listen you'll hear the false yam speak, quietly, I yam not a yam. Only the true yam will identify itself. Lean close and let it whisper gentle into that good ear: I yam what I yam, and that is all what I yam (except what I yain't).