1 Shakespeare: {by Ophelia's desk} I found a clever thing on the net. The legendary Helen had a "face that launched a thousand ships", so a millihelen is a unit of beauty sufficient to launch one ship.
2 Ophelia: Oh Will, that's so old. Everyone knows that. And it's silly too.
3 Mercutio: {walking up to join the conversation} That's not the real problem with it though.
3 Ophelia: Oh, and what's that?
4 Mercutio: You can't mix metric prefixes with Troy units like that.

Helen. Troy.

Incidentally, the line about launching a thousand ships comes from this:

Faustus

Was this the face that launch'd a thousand ships,
And burnt the topless towers of Ilium?--
Sweet Helen, make me immortal with a kiss.

This is from The Tragical History of Doctor Faustus, a play published in 1604, by William Shakespeare's contemporary, Christopher Marlowe, after whom Will, Ophelia, and Mercutio's boss is named.

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2015-01-10 Rerun commentary: The helen as a unit of beauty has been around for a long time. The origin seems uncertain, but according to Wikipedia's article on humorous units of measurement, it was possibly first suggested by none other than Isaac Asimov, prior to 1981.

I'm more concerned about those "topless towers of Ilium", though. How is a tower topless? If it doesn't have a top, then that would imply that it goes up forever.

EDIT: Well, naturally several readers chimed in with suggestions:

1. The towers have bare breasts. One reader supposed this might be a deliberate taunt to the Greeks, being built that way as a reference to Helen.
2. The towers might actually be space elevators. This seems a little high-tech for Troy to me, but I suppose with the help of Atlantis they might have managed it.
3. The towers used to have tops, but they fell down or were destroyed, leaving a jagged unfinished upper portion without a true "top", as it were.
4. The phrase "topless towers" may be a poetic way of saying that there are no other towers that surpass them - in height most probably, but perhaps also in other respects, sort of like "matchless".
5. Or perhaps it was just literary hyperbole. Naahhh...