My Babysitter's a Vampire Wiki
"Benny:Why is she always so na na na na na na?
Erica: I heard that!
Benny: Stupid Vampire Super Hearing.
Benny to Ethan, Erica is overhearing him[src]

Erica after she hears Benny.

Superhuman Hearing is an ability used by Vampires and Fledglings to listen in on conversations. That is how they know most stuff, because of super hearing. Erica overheard Benny when he was saying "Why is she always so, na, na, na, na, na na"?