A float has to float

Attempting to improve on my ability to craft dialogue, I find myself listening to people more and more, eavesdropping on conversations and taking careful note of a person's choice of words.  Today I was in Carvel, waiting to order, when the woman in front of me was handed her root beer float.  She looked at it, paused a moment and then asked, "Don't you mix these up?"

Obviously, the woman was a lunatic to assume that a root beer float should be mixed like a shake.  The word float implies...