What's the deal with Buttercups?

I never got the whole deal with buttercups. That thing about if one is placed under your chin and then makes it glow yellow then you like butter (wow! light reflection ooooh!)
Where the hell did that come from? Parents wanting their little angels to like butter maybe? Who needs to be told whether they like butter anyway? Can't you just try it?

I wonder if it works with other food? If a cocoa bean makes my chin brown does that mean I like chocolate?