The secret which Joss and Tim were "laying in" to develop it later.
I think i know what it is.
Clues:
- syringe (pointed out by Joss)
- "Don't wanna die at all" (pointed out by Tim)
- "You look excatly like the day I left" (pointed out by me humble self, yay!)
So, obviously, she uses some injections to stay youthful.
The question left is this: is she as young as we think she is, or is she, like, 80 y.o. already?