What is the difference between a sweet potato and a yam?

Who's asking: Kathy Henry, Hallowell, ME

Americans use the terms "sweet potato" and "yam" more or less interchangeably, but they're not the same thing at all.

We don't have much opportunity to be able to tell the difference, because it's rare to find a yam in an American food market. What we call yams are sweet potatoes with bright orange flesh; other varieties of sweet potato have lighter, yellowish flesh.

Yams are a staple in Caribbean and African cooking. (Yams are native to Africa and Asia, while sweet potatoes are native to South America.) They can be enormous -- up to seven feet long -- and can be almost any color, from off-white to purple. Yams are starchy, while sweet potatoes are -- uh -- sweet. Sweet potatoes belong to the morning glory family, but yams are their own family (Dioscoreaceae). Sweet potatoes will grow in both tropical and temperate zones, but yams need tropical weather. Sweet potatoes are high in beta carotene, but yams are not a particularly good source of this nutrient.

It's supposed to be the first full day of spring, but the thermometer says 15F. I know I complained about how late winter was this year, but I'm ready to move on now.

Five Random Songs

"Follow You, Follow Me," Genesis. A great love song, from the CD my friends Brian and Scott gave to their wedding guests.

"Chimes of Freedom," The Byrds.

"(Don't Go Back to) Rockville," R.E.M. It's been 23 years since Reckoning was the coolest record in anyone's collection. How is this possible?

"Every Time it Rains," Howard Tate. Howard Tate is a classic, old-school R&B singer who's making a comeback with this CD, Portrait of Howard, produced by my old friend Steve Weisberg. Anyone who loves Al Green or Jackie Wilson should make a point of picking up this record.

"Dead," They Might Be Giants. A catalog of regrets from someone who's just died: "I will never say the word/"Procrastinate" again; I'll never/See myself in the mirror with my eyes closed/I didn't apologize for/When I was eight and I made my brother/Have to be my personal slave."