Light Brown Sugar vs. Dark Brown Sugar: What's the Difference?

Light brown sugar versus dark brown sugar. I've always wondered about this, and I just never bothered to ask or look it up.

UNTIL TODAY.

So basically, your brown sugar is a combination of regular sugar and molasses. 

Who knew? 

The molasses is what darkens the sugar.

So your light brown sugar doesn't have as much (as many?) molasses, and your dark brown sugar does.

Which one should you use? Are they interchangeable?

From what I read, it sounds like it comes down to taste. If you use the dark brown sugar, you're going to have a stronger molasses taste, and if you use the light one, you'll have a lighter taste, so there you go.

I'd say follow the recipe. If it calls for dark, use dark. If it calls for light, use light. But in a pinch, it sounds like you probably can get away with swapping one for the other, if necessary. Just know it might affect the taste a little. Or a lot.

I sound like Julia Childs, don't I? Wait. Is it Child or Childs? I don't even know that. 

[Editor's note: Child]