From the episode "Galaxy's Child"

GEORDI
This is a space baby, right? I
mean, these creatures are born,
live, and die in interstellar
space?

LEAH
Apparently...

GEORDI
Okay. All matter in space
vibrates in a specific radiation
band...

LEAH
Twenty-one centimeters...

Suddenly she gasps involuntarily, lifts her hands to
her cheeks, breaks into a big smile.

LEAH
That's good, Commander, that's
very good. We could alter the
power frequency so it's completely
foreign to the lifeform's natural
vibrations...

GEORDI
And that might be just enough to
pour a little vinegar in baby's
milk.