Gravity Falls, Season 1

[Stan discovers Mabel (in Dipper's body) watching Dipper (in her body) hanging out with her friends]

Stan:
What's goin' on, Dipper? Ah, you're at that creepy age where you spy on girls, huh? Guess it's time you and me had a man-to-man talk - about the birds and the bees, you know?

[Mabel looks mortified. Cut to Stan opening a book called "Why Am I Sweaty?"]

Mabel:
I—I should really be going--

Stan:
No way out of it! Look, it all begins with this little fella, the pituitary gland. [grinning] He may be little, but he has BIG PLANS.

Mabel:
AAAAAAAAH!!!

Stan:
[later; closing the book] And now you know where babies come from.

Mabel:
[traumatized] Goodbye, childhood...