Because Voldemort farted, Harry Potter lost the secret key.

Yes all you Harry Potter fans! - there was something you all missed while reading the book! There was a secret key, which was blown away (possibly incinerated in Harry's hands) by a fart from He-Who-Must-Not-Be-Named!
No I am just kidding. It was just something on my homework:

Add an adverbial clause to the following:
Harry Potter lost the secret key.
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Now the substitute teacher told us that we could write anything - our regular teacher (Ms. O'neil) apparently said so. Now in class, one "genius" named Chris came up with this sentence:

Beacause of Lord Voldemort, Harry Potter lost the secret key.

The next day, Ms.O'neil was back, and we went through all the sentences that everybody made. And she explained on the chalkboard with flourish why Chris's sentence is a big "NO-NO" (yes, that's the word she used).

"NOW what WOULD work is THIS...! *erase* *write*
Because Lord Voldemort farted, Harry Potter lost the secret key.

And for whatever reason, the entire class erupted into laughter. And maybe it was only funny because she started cracking jokes.

My favorite one I did (which received one of the few eruptions of laughter)

Add a sentence to the following adverbial clause:
When the moon is full...
Dracula and the werewolves will have a disco party.