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.